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SubjectRe: [PATCH] can: mcp251x: convert to half-duplex SPI
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On 5/25/20 2:57 PM, Mark Brown wrote:
> On Mon, May 25, 2020 at 02:41:31PM +0200, Marc Kleine-Budde wrote:
>> On 5/25/20 1:31 PM, Mark Brown wrote:
>
>>> This isn't something that every individual driver should be doing, such
>>> rewriting should happen in the core so that everything sees the benefit.
>
>> The core could merge several half duplex transfers (until there's as cs_change)
>> into a single full duplex transfer.
>
> Yes, that is what I am suggesting.

Where in the SPI stack do you see such a "merge" function? One point to clarify
is when and where to allocate and free the memory for the contiguous full duplex
buffers.

>> I think it's not easy to detect and reliable to split a full duplex transfer
>> into half duplex ones. How can you tell, if the controller is supposed to tx 0x0
>> or actually receive.
>
> I don't understand how that could possibly work or why it would make
> sense?

ACK, I was just thinking loud about options.

>> I think spi_write_then_read() can be extended to generate one full duplex
>> transfer instead on two half duplex ones it does a memcpy() anyways.
>
> This has the same problem as doing it in any other driver code - it
> causes a needless incompatibility with three wire and single duplex
> devices.

What about the note "portable code should never use this for more than 32 bytes"
in spi_write_then_read()? The CAN driver in question may read more than 32 bytes
of data.

>> To get a feeling for the use cases, this is what I do in the regmap read
>> function of a (not yet mainlined) CAN SPI driver.
>
> Like I say it's probably better if code like this gets pushed into the
> SPI core where we've got more information about what the controller can
> do and there's more win from doing the tuning since more devices and
> systems can take advantage of it.

ACK

Marc

--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |

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