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

[Marek Behún]

On Mon, 3 Oct 2022 15:11:44 +0100
"Russell King (Oracle)" <linux at armlinux.org.uk> wrote:

> 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?
> 

Hello Russell,

thanks for your suggestion!

Adding Pali, since he has some information (see at the end of this
message).

The attached patch seems to solve the null-pointer dereference.
I booted into single user mode and enabled eth2. Before it caused the
NULL pointer dereference after link got up, not it does not happen.

But I am still encountering the freeze after booting into system.

Maybe these are different bugs?

I am thinking whether we don't need something similar like
  7bea67a99430 ("ARM: dts integrator: Fix DMA ranges")
also for mvebu.
I seem to remember Pali talking about how the ranges defined in some
upstream mvebu-tree, using MBUS_ID() macros, are incorrect.
Pali, what do you remember about this?

Marek
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