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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