Re: [PATCH v1 1/1] spi: Remove unneded check for orig_nents

[Nícolas F. R. A. Prado]

On Thu, May 16, 2024 at 04:25:35PM +0300, Andy Shevchenko wrote:
> On Thu, May 16, 2024 at 01:18:04PM +0300, Andy Shevchenko wrote:
> > On Wed, May 15, 2024 at 05:09:33PM -0400, Nícolas F. R. A. Prado wrote:
> > > On Tue, May 07, 2024 at 11:10:27PM +0300, Andy Shevchenko wrote:
> > > > Both dma_unmap_sgtable() and sg_free_table() in spi_unmap_buf_attrs()
> > > > have checks for orig_nents against 0. No need to duplicate this.
> > > > All the same applies to other DMA mapping API calls.
> > > > 
> > > > Also note, there is no other user in the kernel that does this kind of
> > > > checks.
> > > > 
> > > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > 
> > > this commit caused a regression which I reported here:
> > > 
> > > https://lore.kernel.org/all/d3679496-2e4e-4a7c-97ed-f193bd53af1d@notapiano
> > > 
> > > along with some thoughts on the cause and a possible solution, though I'm not
> > > familiar with this code base at all and would really appreciate any feedback you
> > > may have.
> > 
> > Thanks for the report and preliminary analysis!
> > I'll look at it hopefully sooner than later.
> > 
> > But at least what I think now is that my change revealed a problem somewhere
> > else, because that's how DMA mapping / streaming APIs designed, it's extremely
> > rare to check orig_nents field.
> 
> Can you test the below patch?
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index b2efd4964f7c..51811f04e463 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1243,6 +1243,7 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
>  	else
>  		rx_dev = ctlr->dev.parent;
>  
> +	ret = -ENOMSG;
>  	list_for_each_entry(xfer, &msg->transfers, transfer_list) {
>  		/* The sync is done before each transfer. */
>  		unsigned long attrs = DMA_ATTR_SKIP_CPU_SYNC;
> @@ -1272,6 +1273,9 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
>  			}
>  		}
>  	}
> +	/* No transfer has been mapped, bail out with success */
> +	if (ret)
> +		return 0;
>  
>  	ctlr->cur_rx_dma_dev = rx_dev;
>  	ctlr->cur_tx_dma_dev = tx_dev;

Hi Andy,

thank you for the patch. Unfortunately it didn't completely solve the issue. Now
the stack trace is slightly different and points at the next line:

	dma_sync_sgtable_for_device(rx_dev, &xfer->rx_sg, DMA_FROM_DEVICE);

So now we're hitting the case where only the tx buffer was DMA mapped, but the
rx is still uninitialized, though the cur_msg_mapped flag is set to true, since
it is shared between them. The original code checked for the initialization of
each scatterlist individually, which is why it worked.

Thanks,
Nícolas