linux-kernel - Re: [PATCH] mmc: core: Only set maximum DMA segment size if DMA is supported

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Message-ID: <c9100637-663a-4307-9d59-deebcf9ad34c@arm.com>
Date: Thu, 26 Sep 2024 16:56:15 +0100
From: Robin Murphy <robin.murphy@....com>
To: Guenter Roeck <linux@...ck-us.net>, Ulf Hansson <ulf.hansson@...aro.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Samuel Holland <samuel.holland@...ive.com>, linux-mmc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
 Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] mmc: core: Only set maximum DMA segment size if DMA is
 supported

On 24/09/2024 10:01 pm, Guenter Roeck wrote:
> Since upstream commit 334304ac2bac ("dma-mapping: don't return errors
> from dma_set_max_seg_size") calling dma_set_max_seg_size() on a device
> not supporting DMA results in a warning traceback. This is seen when
> booting the sifive_u machine from SD. The underlying SPI controller
> (sifive,spi0 compatible) explicitly sets dma_mask to NULL.

Hmm, that driver probably shouldn't be doing that, but either way it's 
not actually relevant to this check - what's significant is that when 
the MMC host device is an "mmc-spi-slot" instance, its mmc_dev() ends up 
being the SPI *device* itself, not the grandparent SPI controller.

Thus the patch itself is appropriate on the basis that MMC hosts may 
exist on non-DMA-capable buses, so the subsystem needs to consider that.

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Avoid the backtrace by only calling dma_set_max_seg_size() if DMA is
> supported.
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Ulf Hansson <ulf.hansson@...aro.org>
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
>   drivers/mmc/core/queue.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
> index d0b3ca8a11f0..4d6844261912 100644
> --- a/drivers/mmc/core/queue.c
> +++ b/drivers/mmc/core/queue.c
> @@ -388,7 +388,8 @@ static struct gendisk *mmc_alloc_disk(struct mmc_queue *mq,
>   
>   	blk_queue_rq_timeout(mq->queue, 60 * HZ);
>   
> -	dma_set_max_seg_size(mmc_dev(host), queue_max_segment_size(mq->queue));
> +	if (mmc_dev(host)->dma_parms)
> +		dma_set_max_seg_size(mmc_dev(host), queue_max_segment_size(mq->queue));
>   
>   	INIT_WORK(&mq->recovery_work, mmc_mq_recovery_handler);
>   	INIT_WORK(&mq->complete_work, mmc_blk_mq_complete_work);