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

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Date:   Fri, 2 Jun 2017 07:38:52 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Jiada Wang <jiada_wang@...tor.com>
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 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.

Sascha

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