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

From: Tony Lindgren <[email-masked]>
Date: Thu, 29 Nov 2018 11:13:32 -0800

* Ulf Hansson <[email-masked]> [181119 12:09]:
> On 31 October 2018 at 16:57, Tony Lindgren <[email-masked]> wrote:
> > With CONFIG_DMA_API_DEBUG_SG a device may produce the following warning:
> >
> > "DMA-API: mapping sg segment longer than device claims to support"
> >
> > We default to 64KiB if a DMA engine driver does not initialize dma_parms
> > and call dma_set_max_seg_size(). This may be lower that what many MMC
> > drivers do with mmc->max_seg_size = mmc->max_blk_size * mmc->max_blk_count.
> >
> > Let's do a sanity check for max_seg_size being higher than what DMA
> > supports in mmc_add_host() and lower it as needed.
> >
> > Cc: Kishon Vijay Abraham I <[email-masked]>
> > Cc: Peter Ujfalusi <[email-masked]>
> > Cc: Russell King <[email-masked]>
> > Reported-by: Russell King <[email-masked]>
> > Signed-off-by: Tony Lindgren <[email-masked]>
> > ---
> >  drivers/mmc/core/host.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
> > --- a/drivers/mmc/core/host.c
> > +++ b/drivers/mmc/core/host.c
> > @@ -13,6 +13,7 @@
> >   */
> >
> >  #include <linux/device.h>
> > +#include <linux/dma-mapping.h>
> >  #include <linux/err.h>
> >  #include <linux/idr.h>
> >  #include <linux/of.h>
> > @@ -415,6 +416,19 @@ struct mmc_host *mmc_alloc_host(int extra, struct device *dev)
> >
> >  EXPORT_SYMBOL(mmc_alloc_host);
> >
> > +static void mmc_check_max_seg_size(struct mmc_host *host)
> > +{
> > +       unsigned int max_seg_size = dma_get_max_seg_size(mmc_dev(host));
> 
> Is dma_get_max_seg_size() really intended to be called for any struct
> device (representing the mmc controller) like this?
> 
> My understanding is that the dma_get_max_seg_size() is supposed to be
> called by using the DMA engine device, no?

Oh good catch sounds like I'm calling it for the wrong device,
need to check. In that case sounds like this can't be generic?

Regards,

Tony

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