Re: USB mass storage and ARM cache coherency

From: Sergei Shtylyov
Date: Fri Jan 29 2010 - 12:53:56 EST


Catalin Marinas wrote:

I've been trying for some time to use a rootfs (ext2) on a USB memory
stick on ARM platforms but without any success. The USB HCD driver is
ISP1760 which doesn't use DMA.

ARM has a Harvard cache architecture and what I get is incoherency
between the I and D caches. The CPU I'm using (ARM11MPCore) has PIPT
caches with D-cache lines allocation on write.

Basically, when user space tries to execute from a new page, it faults
and the data is requested via the VFS layer, SCSI block device and USB
mass storage from the ISP1760 driver. The page is then mapped into user
space and update_mmu_cache() called.

However, since the driver is PIO, the data copied from the USB device
into RAM gets stuck in the D-cache. On the above page requesting path
there is no call to flush_dcache_page() to handle D-cache maintenance
(for DMA drivers, that's handled by the DMA API).

Since the USB mass storage code has the information about the USB driver
Sorry, I am a little confused that usb mass storage has what information
about DMA or PIO of low level usb transfer?

I was thinking about checking dev->bus->controller->dma_mask which the
code (though not the storage one) seems to imply that if the dma_mask is
0, the HCD driver is only capable of PIO.

That would be a more general solution rather than going through each HCD
driver since my understanding is that flush_dcache_page() is only needed
together with the mass storage support.

Note that DMA capable driver can be doing some transfers in PIO mode or falling back to PIO mode if DMA mode transfer is unsuccessful (the musb driver is an example of the latter and if the DMA rewrite patches will get accepted, it'll do short transfers in PIO mode).

MBR, Sergei

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