Re: [PATCH] spi: dw: Fix wrong FIFO level setting for long xfers

[Andy Shevchenko <andy.shevchenko@gmail.com>]

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.

-- 
With Best Regards,
Andy Shevchenko