Allow unaligned input/output to SPI::transferBytes #5709
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Fixes esp8266#4967 Support any alignment of input and output pointers and transfer lengths in SPI::transferBytes. Use 32-bit transfers and FIFO as much as possible.
earlephilhower
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Code is done, but need to do some testing (thinking of a wrapper that makes aligned/misaligned buffers at random). Feel free to test on your own on non-critical data, of course. |
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Can some of the code pieces be factored out? e.g.:
transmitMisaligned()
transmitAligned()
receiveMisaligned()
receiveAligned()
then implement something like:
void trasmit(out)
{
if(out & 3)
transmitMisaligned();
else
transmitAligned();
}
same for receive(in).
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I realized after testing that you can't do bytewise updates to the FIFO space because it's RmW. When you read, you're not reading what was writen by the core but the SPI HW read FIFO. So the original refactoring could not work. Rewrote it with a small shim with handles alignment and then calls the existing transfer operation with aligned input and output buffers, and then handles unaligning output as needed. |
The SPI FIFO can't properly do RMW (i.e. bytewise updates) because when you read the FIFO you are actually reading the SPI read data, not what was written into the write FIFO. Refactor the transferBytes to take account of this. For aligned input and outputs, perform as before (but handle non-x4 sizes properly). For misaligned inputs, if it's unidirectional then do bytewise until the direction data pointer is aligned and then do 32b accesses. Fod bidirectional and misaligned inputs, copy the output data to an aligned buffer, do the transfer, then copy the read back data from temp aligned buffer to the real input buffer.
earlephilhower
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Testing with SDFS and forcing both misaligned read and write buffers as well as no-by-4 lengths, looks good in all testing I can throw at it. |
earlephilhower
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Add more comments and adjust naming to be more informative in transferBytes_ and *aligned_. Save 64bytes of stack in double misaligned case.
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On any misaligned input or output, always use a temp buffer. No need for special casing and bytewise ::transfer(). This should be faster as bytewise ::transfer involves a significant number of IO register accesses and setup. Thanks to @devyte for the suggestion.
Fixes #4967
Support any alignment of input and output pointers and transfer lengths
in SPI::transferBytes. Use 32-bit transfers and FIFO as much as
possible.