spi: dw: Fix wrong FIFO level setting for long xfers

Due to using the u16 type in the min_t() macros the SPI transfer length
will be cast to word before participating in the conditional statement
implied by the macro. Thus if the transfer length is greater than 64KB the
Tx/Rx FIFO threshold level value will be determined by the leftover of the
truncated after the type-case length. In the worst case it will cause
having the "Tx FIFO Empty" or "Rx FIFO Full" interrupts triggered on each
word sent/received to/from the bus. In its turn it will cause the
dramatical performance drop.

The problem can be easily fixed by using the min() macros instead of
min_t() which doesn't imply any type casting thus preventing the possible
data loss.

Fixes: ea11370fffdf ("spi: dw: get TX level without an additional variable")
Reported-by: Sergey Nazarov <Sergey.Nazarov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
---
 drivers/spi/spi-dw-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Fri, Jan 13, 2023 at 6:57 PM Serge Semin
<Sergey.Semin@baikalelectronics.ru> wrote:
>
> Due to using the u16 type in the min_t() macros the SPI transfer length
> will be cast to word before participating in the conditional statement
> implied by the macro. Thus if the transfer length is greater than 64KB the
> Tx/Rx FIFO threshold level value will be determined by the leftover of the
> truncated after the type-case length. In the worst case it will cause
> having the "Tx FIFO Empty" or "Rx FIFO Full" interrupts triggered on each
> word sent/received to/from the bus. In its turn it will cause the
> dramatical performance drop.
>
> The problem can be easily fixed by using the min() macros instead of
> min_t() which doesn't imply any type casting thus preventing the possible
> data loss.

But this would be problematic if the types of the parameters are different.
Currently they are u32 vs. unsigned int. I would rather assume that
FIFO length is always less than or equal to 64K and just change the
type in min_t to follow what dws->tx_len is.

On Fri, Jan 13, 2023 at 09:18:54PM +0300, Serge Semin wrote:
> On Fri, Jan 13, 2023 at 07:33:16PM +0200, Andy Shevchenko wrote:
> > On Fri, Jan 13, 2023 at 6:57 PM Serge Semin
> > <Sergey.Semin@baikalelectronics.ru> wrote:
> > >
> > > Due to using the u16 type in the min_t() macros the SPI transfer length
> > > will be cast to word before participating in the conditional statement
> > > implied by the macro. Thus if the transfer length is greater than 64KB the
> > > Tx/Rx FIFO threshold level value will be determined by the leftover of the
> > > truncated after the type-case length. In the worst case it will cause
> > > having the "Tx FIFO Empty" or "Rx FIFO Full" interrupts triggered on each
> > > word sent/received to/from the bus. In its turn it will cause the
> > > dramatical performance drop.
> > >
> > > The problem can be easily fixed by using the min() macros instead of
> > > min_t() which doesn't imply any type casting thus preventing the possible
> > > data loss.
> 
> > But this would be problematic if the types of the parameters are different.
> > Currently they are u32 vs. unsigned int.
> 
> Yes, it would but only in case if somebody changes their types. As you
> said they are currently of u32 and unsigned int types which are the
> same on all the currently supported platforms. So even if somebody
> changes the type of any of them then the compiler will warn about it
> anyway.
> 
> > I would rather assume that
> > FIFO length is always less than or equal to 64K and just change the
> > type in min_t to follow what dws->tx_len is.
> 
> There is no need in assuming in this case. FIFO depth doesn't exceed
> 256 xfer words by the DW SSI IP-core design (judging by the constraints
> applied to the SSI_RX_FIFO_DEPTH and SSI_TX_FIFO_DEPTH synthesize
> parameters). So the dws->fifo_len can be easily converted to u16 type.
> The problem is in the tx_len field casting to u16. It's a rare case,
> but the SPI xfers length can be greater than 64K. The
> spi_transfer.len field is of the unsigned int type and the SPI-core
> doesn't have any constraints to that (except the one defined by the
> controller drivers).
> 
> So to make sure I correctly understand what you meant. Do you suggest
> to do something like this (it was my first version of the fix):
> -	level = min_t(u16, dws->fifo_len / 2, dws->tx_len);
> +	level = min_t(u32, dws->fifo_len / 2, dws->tx_len);
> or even like this
> -	level = min_t(u16, dws->fifo_len / 2, dws->tx_len);
> +	level = min_t(typeof(dws->tx_len), dws->fifo_len / 2, dws->tx_len);
> ?

No, I suggest

	level = min_t(unsigned int, dws->fifo_len / 2, dws->tx_len);

So, we do not care about changing of the fifo_len type, and we won't issue
a compiler warning if it becomes, let's say, u8. While your solution will
still produce it.

> Personally I would prefer either my solution with just min() macros
> usage (which in case of the types change will give the compile-time
> warning about the types mismatch) or using the min_t(u32, ...) version
> (using typeof() seems overkill). I don't see much different (do you?).

Yes, hence personally I prefer my proposal.

> Both versions have their pros and cons.

Right.