linux-kernel - Re: [PATCH linux-next v3 1/1] spi: imx: dynamic burst length adjust for PIO mode

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Date:   Thu, 8 Jun 2017 22:09:07 -0700
From:   Jiada Wang <jiada_wang@...tor.com>
To:     Sascha Hauer <s.hauer@...gutronix.de>
CC:     <broonie@...nel.org>, <linux-spi@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <festevam@...il.com>
Subject: Re: [PATCH linux-next v3 1/1] spi: imx: dynamic burst length adjust
 for PIO mode

On 06/01/2017 10:38 PM, Sascha Hauer wrote:
> On Thu, Jun 01, 2017 at 04:58:47AM -0700, Jiada Wang wrote:
>> Hi Sascha
>>
>> On 05/29/2017 02:50 AM, Sascha Hauer wrote:
>>> Hi,
>>>
>>> On Thu, May 25, 2017 at 10:02:42PM -0700, jiada_wang@...tor.com wrote:
>>>> From: Jiada Wang<jiada_wang@...tor.com>
>>>>
>>>> previously burst length (BURST_LENGTH) is always set to equal
>>>> to bits_per_word, causes a 10us gap between each word in
>>>> transfer, which significantly affects performance.
>>>>
>>>> This patch uses 32 bits transfer to simulate lower bits transfer,
>>>> and adjusts burst length runtimely to use biggeest burst length
>>>> as possible to reduce the gaps in transfer for PIO mode.
>>>>
>>> First let me say that I'm not really looking forward to have this patch
>>> in the driver. It adds quite some code to already hairy code pathes in
>>> the imx-spi driver and I saw you have the same patch for DMA mode
>>> aswell.
>>>
>>> The driver has different function hooks for the different controllers.
>>> This patch breaks that. In some places it assumes that dynamic burst
>>> is only possible on i.MX51 type controllers and also that in case
>>> dynamic burst is enabled it must be an i.MX51 type controller.
>>>
>>> We should really see how this patch can be better integrated into the
>>> driver, or, how the driver can be changed to better support the dynamic
>>> burst usecase.
>>   Yes, I can understand your concern, as this patch brings in a bunch of
>> change,
>> and changes the behaviour of data transfer.
>> how about introduce a new DTS property like "fsl,spi-dynamic-burst",
>> and only enables dynamic burst when this property is added.
> This will only reduce the testing coverage of the feature, not a good
> option.
As an improvement, I am thinking about the following
1. introduce a 'fsl,spi-dynamic-burst' property in dts as mentioned before
2. remove all change in specific functions hooks, for example 
"mx51_ecspi_config"
3. introduce spi_imx_pio_dynamic_burst_transfer(), which will handle 
actual swapped data transfer,
     and burst length adjustment.


What do you think ?
> Sascha
>