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

-- 
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