Re: [PATCH 1/2] mwifiex: Use non-posted PCI register writes

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Keith]

On Wed, Sep 01, 2021 at 05:41:56PM -0500, Bjorn Helgaas wrote:
> On Wed, Sep 01, 2021 at 07:07:58PM +0200, Johannes Berg wrote:
> > On Wed, 2021-09-01 at 18:51 +0200, Heiner Kallweit wrote:
> > > On 01.09.2021 17:51, Pali Rohár wrote:
> > > > On Wednesday 01 September 2021 16:01:54 Jonas Dreßler wrote:
> > > > > On 8/30/21 2:49 PM, Andy Shevchenko wrote:
> > > > > > On Mon, Aug 30, 2021 at 3:38 PM Jonas Dreßler <verdre@v0yd.nl> wrote:
> > > > > > > 
> > > > > > > On the 88W8897 card it's very important the TX ring write pointer is
> > > > > > > updated correctly to its new value before setting the TX ready
> > > > > > > interrupt, otherwise the firmware appears to crash (probably because
> > > > > > > it's trying to DMA-read from the wrong place).
> > > > > > > 
> > > 
> > > This sounds somehow like the typical case where you write DMA descriptors
> > > and then ring the doorbell. This normally requires a dma_wmb().
> > > Maybe something like that is missing here?
> > 
> > But it looks like this "TX ring write pointer" is actually the register?
> > 
> > However, I would agree that doing it in mwifiex_write_reg() is possibly
> > too big a hammer - could be done only for reg->tx_wrptr, not all the
> > registers?
> > 
> > Actually, can two writes actually cross on PCI?
> 
> Per PCIe r5.0, sec 2.4.1,
> 
>   A2a  A Posted Request must not pass another Posted Request unless A2b
>        applies.
> 
>   A2b  A Posted Request with RO Set is permitted to pass another
>        Posted Request.  A Posted Request with IDO Set is permitted to
>        pass another Posted Request if the two Requester IDs are
>        different or if both Requests contain a PASID TLP Prefix and
>        the two PASID values are different.
> 
> A few drivers enable RO (Relaxed Ordering) for their devices, which
> means the *device* is permitted to set the RO bit in transactions it
> initiates.
> 
> BUt IIUC we're talking about MMIO writes initiated by a CPU, and they
> won't have the RO bit set unless the Root Port has Relaxed Ordering
> enabled, and Linux generally does not enable that.  So A2a should
> apply, and writes should be ordered on PCI.
> 
> There are a few wrinkles that I worry about:
> 
>   d1e714db8129 ("mtip32xx: Fix ERO and NoSnoop values in PCIe upstream
>   on AMD systems") [1] turns off RO for some AMD Root Ports, which
>   makes me think BIOS might be enabling RO in these Root Ports.
> 
>   c56d4450eb68 ("PCI: Turn off Request Attributes to avoid Chelsio T5
>   Completion erratum") [2] turns off RO for all Root Ports leading to
>   Chelsio T5 devices, which again makes me think there's firmware that
>   enables RO in Root Ports.  Follow-up [3].
> 
>   77ffc1465cec ("tegra: add PCI Express support") [4] (see
>   tegra_pcie_relax_enable()) enables RO for Tegra Root Ports due to
>   some hardware issue.  I don't whether these Root Ports every
>   actually *set* RO in the PCIe transactions they generate.  Follow-up
>   [5].
> 
> These concern me because I don't think we have a way for drivers to
> specify whether their writes should use strong ordering or relaxed
> ordering, and I think they depend on strong ordering.

Sorry, I was completely wrong about this part.  Keith Busch reminded
me that writel_relaxed() is an obvious way for drivers to specify that
they don't need strong ordering.

I apologize for the distraction.

> If Root Ports have RO enabled, I think we are at risk, so I suspect
> Linux should actively *disable* RO for Root Ports.
> 
> [1] https://git.kernel.org/linus/d1e714db8129
> [2] https://git.kernel.org/linus/c56d4450eb68
> [3] https://lore.kernel.org/r/20210901222353.GA251391@bjorn-Precision-5520
> [4] https://git.kernel.org/linus/77ffc1465cec
> [5] https://lore.kernel.org/r/20210901204045.GA236987@bjorn-Precision-5520