REGRESSION in 6.0-rc7 caused by patch "ARM/dma-mapping: use dma-direct unconditionally"

[Russell King (Oracle)]

On Mon, Oct 03, 2022 at 09:30:37AM +0200, Christoph Hellwig wrote:
> On Fri, Sep 30, 2022 at 05:02:05PM +0200, Marek Behún wrote:
> > It seems that the null pointer dereference comes from the data variable
> > having zero value. We assign
> >   data = (u8 *)(uintptr_t)rx_desc->buf_cookie;
> 
> I never see any assignment to ->buf_cookie in the driver, what am
> I missing?

I think Marek's setup (like my setups) use the hardware buffer manager,
and it's hardware that fills in the "buf_cookie", which is supposed to
be the virtual address of the buffer.

Each buffer supplied to the hardware buffer manager is supposed to
contain the virtual address in the first 32-bit word in that buffer.

This is done by mvneta_bm_construct():

        /* In order to update buf_cookie field of RX descriptor properly,
         * BM hardware expects buf virtual address to be placed in the
         * first four bytes of mapped buffer.
         */
        *(u32 *)buf = (u32)buf;

immediately prior to dma_map_single(..., DMA_FROM_DEVICE) is called.

If I had to guess, I would suggest that this write is being lost via
cache invalidation, and given that the hardware BM both reads and
writes this buffer, DMA_FROM_DEVICE is not correct, it should be
DMA_BIDIRECTIONAL.

Changing that is probably going to need DMA_FROM_DEVICE also changed
elsewhere in the mvneta_bm and mvneta driver.

I'm not in a position where I could test that out. Marek?

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