[M] Change in coreboot[master]: soc/amd/common/block/lpc/spi_dma: Leverage CBFS_CACHE when using SPI DMA

[Tim Van Patten (Code Review)]

Attention is currently required from: Jason Glenesk, Raul Rangel, Martin Roth, Fred Reitberger, Karthik Ramasubramanian, Felix Held.

Tim Van Patten has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/74334 )

Change subject: soc/amd/common/block/lpc/spi_dma: Leverage CBFS_CACHE when using SPI DMA
......................................................................

Patch Set 4:

(2 comments)

File src/soc/amd/common/block/lpc/spi_dma.c:

https://review.coreboot.org/c/coreboot/+/74334/comment/dbf29f04_156daa30 
PS4, Line 218: 	if (!CONFIG_CBFS_CACHE_SIZE)
> By extra copy into the cache, are you referring to cbfs_cache?

Correct.

`spi_dma_readat_mmap()` will copy into the allocated cache buffer:

```
static ssize_t spi_dma_readat_mmap(const struct region_device *rd, void *b, size_t offset,
    			   size_t size)
{
    const struct mem_region_device *mdev;

mdev = container_of(rd, __typeof__(*mdev), rdev);

memcpy(b, &mdev->base[offset], size);

return size;
}
```

Do we want this? If so, then we should update the fallback case (see: ref1 below) to perform the same copy, since the data is still available and the cache buffer is still good.

https://review.coreboot.org/c/coreboot/+/74334/comment/332fbd2e_2488bf43 
PS4, Line 230: 		mem_pool_free(&cbfs_cache, mapping);
             : 		/* Fall-back to memory map */
             : 		return &mdev->base[offset];
ref1