Re: [PATCH] mmc: dw_mmc: modified the DMA threshold for SD card

[James Hogan <james.hogan@imgtec.com>]

Hi,

On 09/26/2011 05:24 AM, Jaehoon Chung wrote:
> James Hogan wrote:
> 
>> On 09/22/2011 11:01 AM, Jaehoon Chung wrote:
>>> This patch modified the DMA threshold.
>>>
>>> I didn't know exactly why need this threshold.
>>> But if this value is 16, we didn't read SCR register for SD card.
>>> Because in mmc_app_send_scr this values are used data.blocks=1 and data.blksz=8.
>>> (data.blocks * data.blksz = 8...so return -EINVAL)
>>
>> The idea is that for a small transaction the time to set up the DMA is
>> probably not worth the effort, therefore it does it using PIO mode (see
>> dw_mci_submit_data, if dw_mci_submit_data_dma fails, it sets itself up
>> for pio mode instead).
> 
> Is there some reason that threshold is set 16?

I expect it's fairly arbitrary, and the optimum value would depend on
what DMA is used etc.

> 
>>
>> So if it's using PIO mode and the SCR cannot be read, then PIO mode is
>> broken. What's the value of the register HCON? dw_mci_probe reads it and
>> decides which pio push and pull function to use based on the host data
>> width. I've only tested one of them as we only have one configuration of
>> hardware available, but you may have a different one.
> 
> 
> i used push/pull_data_32. What do you doubt this problem?

Our current hardware also has a data width of 32.

Does it work if you disable DMA (by returning -ENODEV at the beginning
of dw_mci_submit_data_dma)? That would prove that the problem wasn't
caused by DMA mode leaving the hardware in a bad state before switching
to PIO mode.

Cheers
James

> 
> Best regards,
> Jaehon Chung
> 
>