Re: [PATCH] can: mcp251x: convert to half-duplex SPI

[Mark Brown <broonie@kernel.org>]

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On Mon, May 25, 2020 at 03:12:05PM +0200, Marc Kleine-Budde wrote:
> 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:

> >> 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.

My first thought would be about the same point as we're rewriting to
handle MUST_TX and MUST_RX in map_msg() which does similar allocations
and deallocations to insert dummy data for controllers that need them.

> >> 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.

I think that comment is actually not valid any more - we used to use a
fixed statically allocated buffer in write_then_read() but added the
option to fall back onto allocating one dynamically if another user was
running or the transfer was too big.

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