| Message ID | 20220927112117.77599-4-vincent.whitchurch@axis.com |
|---|---|
| State | New |
| Headers | show |
| Series | spi: Fix DMA bugs in (not only) spi-s3c64xx | expand |
On 9/27/22 06:21, Vincent Whitchurch wrote: > A couple of drivers call spi_split_transfers_maxsize() from their > ->prepare_message() callbacks to split transfers which are too big for > them to handle. Add support in the core to do this based on > ->max_transfer_size() to avoid code duplication. Hello, I've been testing AT25 functionality in linux 6.1 and I believe this patch is breaking the AT25 protocol. It will split a write command up such that some of the data is in a different transfer than the write enable and address. According to my understanding of the AT25 spec, that doesn't work... Someone correct me if I'm wrong though. Do we need a flag to enable/disable this behavior depending on the client perhaps? Thanks, Eddie > > Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com> > --- > drivers/spi/spi.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c > index f41a8c2752b8..44e4352d948b 100644 > --- a/drivers/spi/spi.c > +++ b/drivers/spi/spi.c > @@ -1649,6 +1649,15 @@ static int __spi_pump_transfer_message(struct spi_controller *ctlr, > > trace_spi_message_start(msg); > > + ret = spi_split_transfers_maxsize(ctlr, msg, > + spi_max_transfer_size(msg->spi), > + GFP_KERNEL | GFP_DMA); > + if (ret) { > + msg->status = ret; > + spi_finalize_current_message(ctlr); > + return ret; > + } > + > if (ctlr->prepare_message) { > ret = ctlr->prepare_message(ctlr, msg); > if (ret) {
On Thu, Jun 22, 2023 at 02:48:36PM -0500, Eddie James wrote: > On 9/27/22 06:21, Vincent Whitchurch wrote: > > A couple of drivers call spi_split_transfers_maxsize() from their > > ->prepare_message() callbacks to split transfers which are too big for > > them to handle. Add support in the core to do this based on > > ->max_transfer_size() to avoid code duplication. > I've been testing AT25 functionality in linux 6.1 and I believe this patch > is breaking the AT25 protocol. It will split a write command up such that > some of the data is in a different transfer than the write enable and > address. According to my understanding of the AT25 spec, that doesn't > work... Someone correct me if I'm wrong though. Do we need a flag to > enable/disable this behavior depending on the client perhaps? Could you be more specific about the manner in which you think this is breaking things? The size of transfer is immaterial to the client device on SPI, the client will be counting clocks while the chip select is asserted. How the controller chooses to split things up is really not particularly visible or relevant, it might bitbang things out one bit at a time, transfer a single word at a time or batch things up further. So long as the chip select is asserted it's all the same to the client device. In any case this is all based on the maximum transfer size advertised by the conteroller driver, if the device can physically handle larger transfers then there's no reason for it to set a limit. If the driver can't physically handle larger transfers and it does make a difference then the system simply won't work.
On 6/22/23 16:16, Mark Brown wrote: > On Thu, Jun 22, 2023 at 02:48:36PM -0500, Eddie James wrote: >> On 9/27/22 06:21, Vincent Whitchurch wrote: >>> A couple of drivers call spi_split_transfers_maxsize() from their >>> ->prepare_message() callbacks to split transfers which are too big for >>> them to handle. Add support in the core to do this based on >>> ->max_transfer_size() to avoid code duplication. >> I've been testing AT25 functionality in linux 6.1 and I believe this patch >> is breaking the AT25 protocol. It will split a write command up such that >> some of the data is in a different transfer than the write enable and >> address. According to my understanding of the AT25 spec, that doesn't >> work... Someone correct me if I'm wrong though. Do we need a flag to >> enable/disable this behavior depending on the client perhaps? > Could you be more specific about the manner in which you think this is > breaking things? The size of transfer is immaterial to the client > device on SPI, the client will be counting clocks while the chip select > is asserted. How the controller chooses to split things up is really > not particularly visible or relevant, it might bitbang things out one > bit at a time, transfer a single word at a time or batch things up > further. So long as the chip select is asserted it's all the same to > the client device. Ok, I understand better now. Agreed it shouldn't make a difference, but this is actually a limitation of the spi controller I'm using (spi-fsi). The controller cannot handle multiple transfers keeping the chip select enabled... I guess the driver can batch transfers in the message to get around this, unless you want to add a flag for that behavior. > > In any case this is all based on the maximum transfer size advertised by > the conteroller driver, if the device can physically handle larger > transfers then there's no reason for it to set a limit. If the driver > can't physically handle larger transfers and it does make a difference > then the system simply won't work. Yep, this is also an artifact of the spi-fsi driver having different transfer size limits for writes and reads. Funnily enough the at25 driver doesn't truly respect the max transfer size (it doesn't include the write command and address bytes in the calculation against the max transfer size) so that's how this worked previously. Thanks! Eddie
On Fri, Jun 23, 2023 at 11:45:19AM -0500, Eddie James wrote: > On 6/22/23 16:16, Mark Brown wrote: > > On Thu, Jun 22, 2023 at 02:48:36PM -0500, Eddie James wrote: > > > On 9/27/22 06:21, Vincent Whitchurch wrote: > > > > A couple of drivers call spi_split_transfers_maxsize() from their > > > > ->prepare_message() callbacks to split transfers which are too big for > > > > them to handle. Add support in the core to do this based on > > > > ->max_transfer_size() to avoid code duplication. > > > I've been testing AT25 functionality in linux 6.1 and I believe this patch > > > is breaking the AT25 protocol. It will split a write command up such that > > > some of the data is in a different transfer than the write enable and > > Could you be more specific about the manner in which you think this is > > breaking things? The size of transfer is immaterial to the client > Ok, I understand better now. Agreed it shouldn't make a difference, but this > is actually a limitation of the spi controller I'm using (spi-fsi). The > controller cannot handle multiple transfers keeping the chip select > enabled... I guess the driver can batch transfers in the message to get > around this, unless you want to add a flag for that behavior. Client drivers should in general just generate messages corresponding to the desired result visible to the device and let the controller worry about how to actually accomplish that, splitting transfers needlessly is just going to create overheads. If there's scatter/gather going on that does complicate things a bit though so it's not always going to happen. If the controller driver needs to rewrite the message to combine transfers then it should do that (or tell the core to do so on it's behalf), just like with splitting transfers due to length limits. > > In any case this is all based on the maximum transfer size advertised by > > the conteroller driver, if the device can physically handle larger > > transfers then there's no reason for it to set a limit. If the driver > > can't physically handle larger transfers and it does make a difference > > then the system simply won't work. > Yep, this is also an artifact of the spi-fsi driver having different > transfer size limits for writes and reads. Funnily enough the at25 driver > doesn't truly respect the max transfer size (it doesn't include the write > command and address bytes in the calculation against the max transfer size) > so that's how this worked previously. Yes, the logic there looks incorrect. As well as the issue you've identified the driver should really be using spi_max_message_size(), with a controller that can't control chip select effectivley like the FSI driver that'll be the same a the transfer size but other drivers will be able to chain multiple transfers together even if there's limits on the transfer length.
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c index f41a8c2752b8..44e4352d948b 100644 --- a/drivers/spi/spi.c +++ b/drivers/spi/spi.c @@ -1649,6 +1649,15 @@ static int __spi_pump_transfer_message(struct spi_controller *ctlr, trace_spi_message_start(msg); + ret = spi_split_transfers_maxsize(ctlr, msg, + spi_max_transfer_size(msg->spi), + GFP_KERNEL | GFP_DMA); + if (ret) { + msg->status = ret; + spi_finalize_current_message(ctlr); + return ret; + } + if (ctlr->prepare_message) { ret = ctlr->prepare_message(ctlr, msg); if (ret) {
A couple of drivers call spi_split_transfers_maxsize() from their ->prepare_message() callbacks to split transfers which are too big for them to handle. Add support in the core to do this based on ->max_transfer_size() to avoid code duplication. Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com> --- drivers/spi/spi.c | 9 +++++++++ 1 file changed, 9 insertions(+)