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Date: Wed, 15 Nov 2023 14:25:37 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Serge Semin <fancer.lancer@...il.com>
Cc: Jianheng Zhang <Jianheng.Zhang@...opsys.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <Jose.Abreu@...opsys.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Tan Tee Min <tee.min.tan@...el.com>,
	Ong Boon Leong <boon.leong.ong@...el.com>,
	Voon Weifeng <weifeng.voon@...el.com>,
	Mohammad Athari Bin Ismail <mohammad.athari.ismail@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-stm32@...md-mailman.stormreply.com" <linux-stm32@...md-mailman.stormreply.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: stmmac: fix FPE events losing

On Tue, Nov 14, 2023 at 02:59:57PM +0300, Serge Semin wrote:
> On Tue, Nov 14, 2023 at 11:07:34AM +0000, Jianheng Zhang wrote:
> > The 32-bit access of register MAC_FPE_CTRL_STS may clear the FPE status
> > bits unexpectedly. Use 8-bit access for MAC_FPE_CTRL_STS control bits to
> > avoid unexpected access of MAC_FPE_CTRL_STS status bits that can reduce
> > the FPE handshake retries.
> > 
> > The bit[19:17] of register MAC_FPE_CTRL_STS are status register bits.
> > Those bits are clear on read (or write of 1 when RCWE bit in
> > MAC_CSR_SW_Ctrl register is set). Using 32-bit access for
> > MAC_FPE_CTRL_STS control bits makes side effects that clear the status
> > bits. Then the stmmac interrupt handler missing FPE event status and
> > leads to FPE handshake failure and retries.
> > 
> > The bit[7:0] of register MAC_FPE_CTRL_STS are control bits or reserved
> > that have no access side effects, so can use 8-bit access for
> > MAC_FPE_CTRL_STS control bits.
> > 
> > Fixes: 5a5586112b92 ("net: stmmac: support FPE link partner hand-shaking procedure")
> > Signed-off-by: jianheng <jianheng@...opsys.com>
> > ---
> >  drivers/net/ethernet/stmicro/stmmac/dwmac5.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac5.c b/drivers/net/ethernet/stmicro/stmmac/dwmac5.c
> > index e95d35f..7333995 100644
> > --- a/drivers/net/ethernet/stmicro/stmmac/dwmac5.c
> > +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac5.c
> > @@ -716,11 +716,11 @@ void dwmac5_fpe_configure(void __iomem *ioaddr, u32 num_txq, u32 num_rxq,
> >  	u32 value;
> >  
> >  	if (!enable) {
> 
> > -		value = readl(ioaddr + MAC_FPE_CTRL_STS);
> > +		value = readb(ioaddr + MAC_FPE_CTRL_STS);
> 
> Note this may break the platforms which don't support non-32 MMIOs for
> some devices. None of the currently supported glue-drivers explicitly
> state they have such peculiarity, but at the same time the STMMAC-core
> driver at the present state uses the dword IO ops only. For instance
> the PCIe subsystem has the special accessors for such cases:
> pci_generic_config_read32()
> pci_generic_config_write32()
> which at the very least are utilized on the Tegra and Loongson
> platforms to access the host CSR spaces. These platforms are also
> equipped with the DW MACs. The problem might be irrelevant for all the
> currently supported DW MAC controllers implementations though, but
> still it worth to draw an attention to the problem possibility and in
> order to prevent it before ahead it would be better to just avoid
> using the byte-/word- IOs if it's possible.

Yes, this exists for configuration accesses, and is damn annoying
because the read-modify-write of these can end up clearing PCI
status register bits that are W1C.

I've never heard of that problem with MMIO though.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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