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

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On Thu, Feb 23, 2012 at 11:58 PM, Seungwon Jeon <tgih.jun@xxxxxxxxxxx> wrote:
> Will Newton <will.newton@xxxxxxxxx> wrote:
>> 2012/2/23 Seungwon Jeon <tgih.jun@xxxxxxxxxxx>:
>> > 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@xxxxxxxxxxx>
>> > ---
>> >  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;

All the other drivers seem to set max_req_size based on blk_size and
blk_count not seg_size so the current code looks like it may be wrong.

>  - mmc->max_blk_count is set as host->ring_size.
>  - max_seg_size seems to represent max_blk_size.

I believe max_seg_size should be set to a higher value as it appears
to be only relevant for IOMMU-based drivers, possibly the same as
max_req_size.
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