linux-kernel - Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

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Message-ID: <e9f0cb03-06c3-4cea-ba2b-1ca1571a8f31@linaro.org>
Date: Thu, 12 Jun 2025 16:37:39 +0100
From: James Clark <james.clark@...aro.org>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: Arnd Bergmann <arnd@...db.de>, Frank Li <Frank.li@....com>,
 Vladimir Oltean <olteanv@...il.com>, Mark Brown <broonie@...nel.org>,
 linux-spi@...r.kernel.org, imx@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA



On 12/06/2025 4:36 pm, James Clark wrote:
> 
> 
> On 12/06/2025 3:36 pm, Vladimir Oltean wrote:
>> On Thu, Jun 12, 2025 at 03:14:32PM +0100, James Clark wrote:
>>>> FWIW, the XSPI FIFO performance should be higher.
>>>
>>> This leads me to realise a mistake in my original figures. My head 
>>> was stuck
>>> in target mode where we use DMA so I forgot to force DMA in host mode 
>>> to run
>>> the performance tests. The previous figures were all XSPI mode and 
>>> the small
>>> difference in performance could have been just down to the layout of the
>>> code changing?
>>>
>>> Changing it to DMA mode gives figures that make much more sense:
>>>
>>> Coherent (4096 byte transfers): 6534 kbps
>>> Non-coherent:                   7347 kbps
>>>
>>> Coherent (16 byte transfers):    447 kbps
>>> Non-coherent:                    448 kbps
>>>
>>>
>>> Just for comparison running the same test in XSPI mode:
>>>
>>> 4096 byte transfers:            2143 kbps
>>> 16 byte transfers:               637 kbps
>>
>> So to be clear, the 'non-coherent' test was done just with patch 2
>> applied, or also with 3?
> 
> The whole set, and then the non-coherent patch reverted.
> 

And with DMA forced in host mode as a hack.