Re: [PATCH] mmc: core: Lower max_seg_size if too high for DMA

[Peter Ujfalusi <peter.ujfalusi@ti.com>]

Russell,

On 11/12/2018 15.13, Russell King - ARM Linux wrote:
> 
> We're nearing the merge window, and this is a regression that is yet
> to be solved.  It causes a kernel warning with backtrace, so it's
> very annoying.
> 
> The error is:
> 
> omap-dma-engine 4a056000.dma-controller: DMA-API: mapping sg segment longer than device claims to support [len=69632] [max=65536]
> 
> which indicates that we have a SG segment length that exceeds thte
> published maximum segment size for a device - in this case the
> DMA engine device.  The maximum segment size for the DMA engine comes
> from the default per-device setting, in linux/dma-mapping.h, which is
> 64K.
> 
> However, omap_hsmmc sets:
> 
>         mmc->max_blk_size = 512;       /* Block Length at max can be 1024 */
>         mmc->max_blk_count = 0xFFFF;    /* No. of Blocks is 16 bits */
>         mmc->max_req_size = mmc->max_blk_size * mmc->max_blk_count;
>         mmc->max_seg_size = mmc->max_req_size;
> 
> which ends up telling the block layer that we support a maximum segment
> size of 65535*512, so of course it _will_ pass SG lists where a segment
> is longer than 64K.
> 
> The problem here is that the HSMMC driver doesn't take account of the
> DMA engine device's capabilities.
> 
> We have something of an odd situation in that the omap-dma device's
> maximum SG size depends on the "address width" - it's 64K transfers
> of whatever unit "address width" is, so the current implementation of
> per-device parameters doesn't exactly work.  That means the default
> 64K limit at the device-level is reasonable.
> 
> The only thing I can think of doing is adding into omap_hsmmc:
> 
> 	mmc->max_seg_size = min(mmc->max_req_size,
> 				min(dma_get_max_seg_size(host->rx_chan->device->dev),
> 				    dma_get_max_seg_size(host->tx_chan->device->dev)));
> 
> to limit the maximum segment size to that of the device _and_ dma
> engine's capabilities.

Make sense.

> Doing this solves the problem for me.
> 

- Péter

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