[U-Boot] i.MX dcache issues

[Benoît Thébaudeau]

Hi all,

Has anyone tested i.MX25 or i.MX35 with dcache on?

I'm working with U-Boot 2012.04.01 on several custom platforms using these
processors.

With dcache off, everything works fine, but slowly.

With dcache on, it's much faster (e.g. mtest), but it's impossible to read
files through the eSDHC, and U-Boot hangs if trying to ping using the FEC.

Shouldn't mtest disable dcache automatically, then set it back to its original
state once finished? Otherwise, some of its tests to the memory may actually
test the dcache rather than the memory chips connections.

dcache seems to be enabled for mx35pdk, but disabled for spear3, so I'm
wondering if it has been thoroughly tested on mx35pdk.

I have added traces to arch/arm/lib/cache-cp15.c to check that the MMU is
properly initialized with the appropriate addresses, and it is.

Defining CONFIG_SYS_CACHELINE_SIZE to 32 in the board file, which sets
ARCH_DMA_MINALIGN to 32 instead of 64 does not worsen things (as expected with
32-byte cache lines).

Defining CONFIG_MMC_BOUNCE_BUFFER and/or setting the no_snoop option of the
eSDHC driver does not change anything.

Defining DEBUG in mmc.c shows that the 1st mmc_send_cmd following the retry_scr
label uses a cacheline-unaligned size that gets caught by the buffer bouncing
mechanism that reallocs it for nothing since scr has already been allocated with
a cacheline-aligned size. The requested transfer length is left unchanged by
bouncing, but it seems normal for MMC commands, and it should not be an issue as
long as allocated sizes are aligned.

Also, the mmc read commands to free RAM regions seem to work fine, so I'm
wondering if this issue is not caused only by stack-allocated buffers. I'm not
sure the data ending up in the buffers is random when there is an issue: the
pattern f783 appears very often at the position of the expected 55aa DOS
partition marker.

Shouldn't the MMC/eSDHC drivers flush/invalidate the dcache ranges that they use
for DMA operations? Not doing so would explain why stack-allocated buffers are
more affected than buffers in unused RAM areas.

As to the FEC, dcache issues with DMA seem to have been taken care of, so I'll
add traces to the FEC driver to see if there is any cacheline-unaligned buffer
passed in.

These issues could also be related to L2 on i.MX35, but I don't see any code
enabling it.

BTW, why isn't there an enable_caches function in arch/arm/cpu/arm926ejs/cache.c
or arch/arm/cpu/arm926ejs/cpu.c, just like in arch/arm/cpu/arm1136/cpu.c, so
that dcache can be enabled by default if CONFIG_SYS_DCACHE_OFF isn't defined?

Regards,
Benoît