PCI prefetching even without the prefetchable flag

Julien Jemine

Hi,

I'm facing a problem with the PCI on the DM816x evm.

I have a PCI device with 2 BARs. Those BARs are mapped to user-space with mmap(). I then make 32-bits read/write accesses from my driver and from user-space. On x86 platforms, this works perfectly.

However, on the DM816x, the read operations from user-space generate a 64 bytes length read request on the PCI bus (verified using a hardware PCIe analyzer). This seems to happen only from user-space, not from the kernel, so I guess this has something to do with mmap().

Doing 32-bits accesses is important for me because those reads/writes have side effects. Of course the PREFETCHABLE PCI flag is not set.

Any help is welcome,

Julien.

over 13 years ago

0 Vladimir-XID over 13 years ago

TI__Genius 11470 points

Hello,

I will try to address that issue soon. I will let you know, when I have something.

Vladimir

0 Julien Jemine over 13 years ago in reply to Vladimir-XID

Intellectual 270 points

I have found the solution. The problem was that the "non-prefetchable" property of the memory wasn't preserved through the mmap() (only on ARM, on x86 we do not have that issue). The 64-bytes length matches a cache line's length. So I added those flags in the mmap() callback :

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot) | L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_WRITE | L_PTE_DIRTY;

And it works (though I'm not sure all the flags are required) !

0 Vladimir-XID over 13 years ago in reply to Julien Jemine

TI__Genius 11470 points

I am glad that you solved the issue. We will try to respond faster. Sorry for the inconvenience.

Vladimir

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PCI prefetching even without the prefetchable flag