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

["Marek Behún" <kabel@kernel.org>]

On Mon, 3 Oct 2022 23:30:31 +0200
Marcin Wojtas <mw@semihalf.com> wrote:

> Hi Marek,
> 
> 
> pon., 3 paź 2022 o 17:33 Marek Behún <kabel@kernel.org> napisał(a):
> >
> > On Mon, 3 Oct 2022 15:11:44 +0100
> > "Russell King (Oracle)" <linux@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.
> > >  
> 
> I think the DMA_FROM_DEVICE is used rather properly in the RX path of
> the driver - the CPU doesn't access the payload afterward. The BM only
> pushes the pointers back and forth between internal SRAM ('internal
> pool' - BPPI) to DRAM ('external pool' - BPPE) and the descriptors,
> but afaik it should not touch the buffer contents. But maybe somehow
> it affects the coherency and DMA_BIDIRECTIONAL are indeed required...
> About the coherency itself - please see my comment below.
> 
> Another thought - when writing to *buf (memory normal) shouldn't we add a dsb()?
> 
> > > 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.  
> 
> Did you manage to measure performance impact?

I did not measure any performance impacts. But DMA directianlity
within mvneta seems to be a different bug, as Russell replied, and maybe
was just revealed by this.

> I have one overall concern here. On all kinds of A38x-based boards I
> worked on, by default, the firmware set all devices (e.g. network,
> AHCI, XHCI) on MBUS as fully IO cache coherent - it should be
> reflected in the MVNETA_WIN_BASE(w) registers attribute field. Bits
> [15:8] should be set to 0x1D (or 0x1E if there is a second DRAM CS
> used). Can you please try adding 'dma-coherent;' property under the
> 'internal-regs' node?

Yes, adding dma-coherent solves this issue. See other emails. The
proper solution IMO is to default to dma-coherent on the platform,
which can be done in a simple way (I've sent a patch). We want to be
compatible with older device-trees.

Marek

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