RE: [RFC] mmc: dw_mmc: Fix the max_blk_count in IDMAC

[Seungwon Jeon <tgih.jun@samsung.com>]

Will Newton <will.newton@gmail.com> wrote:
> 2012/2/23 Seungwon Jeon <tgih.jun@samsung.com>:
> > Hi all,
> >
> > Even though 1MB is reserved for descriptor table in IDMAC,
> > the dw_mmc host driver is allowed to receive only maximum
> > 128KB block length in one request. This is caused by setting
> > improper max_blk_count. It needs to be e adjusted so that
> > descriptor table is used fully.
> >
> > It is found that the performance is improved with the increased the
> > max_blk_count. In my case, there is 82%(6%) improvement in sequential
> > write(read). Is there anybody who knows the reason about the old setting?
> >
> > Best regards,
> > Seungwon Jeon.
> >
> > Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
> > ---
> >  drivers/mmc/host/dw_mmc.c |    4 ++--
> >  1 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> > index bf3c9b4..af2b901 100644
> > --- a/drivers/mmc/host/dw_mmc.c
> > +++ b/drivers/mmc/host/dw_mmc.c
> > @@ -1804,9 +1804,9 @@ static int __init dw_mci_init_slot(struct dw_mci *host, unsigned int id)
> >  #ifdef CONFIG_MMC_DW_IDMAC
> >                mmc->max_segs = host->ring_size;
> >                mmc->max_blk_size = 65536;
> > -               mmc->max_blk_count = host->ring_size;
> >                mmc->max_seg_size = 0x1000;
> > -               mmc->max_req_size = mmc->max_seg_size * mmc->max_blk_count;
> > +               mmc->max_req_size = mmc->max_seg_size * host->ring_size;
> > +               mmc->max_blk_count = mmc->max_req_size / 512;
> 
> I am not sure this is correct, it seems like most drivers calculate
> max_req_size like this:
> 
> mmc->max_req_size = mmc->max_blk_size * mmc->max_blk_count;

Could you explain the following origin code in dw_mmc?
We need to change like most other drivers?
  mmc->max_req_size = mmc->max_seg_size * mmc->max_blk_count;
  - mmc->max_blk_count is set as host->ring_size.
  - max_seg_size seems to represent max_blk_size.

I think max_req_size means the total transfer size through DMA descriptors.
max_blk_size just is limited to 16 bits for BLKSIZ register. Actually 512 is used.
Host driver doesn't inform block layer of this size.
Also, block layer doesn't utilize max_blk_size.

Thanks,
Seungwon Jeon.
> 
> And indeed in the external DMA case dw_mmc does that too.
> 
> It should probably be safe to set max_seg_size to max_req_size too.
> 

> >  #else
> >                mmc->max_segs = 64;
> >                mmc->max_blk_size = 65536; /* BLKSIZ is 16 bits */
> > --
> > 1.7.0.4
> >
> >
> > --
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