linux-kernel - Re: [PATCH net-next v4 2/9] net: stmmac: xgmac: add more feature parsing from hw cap

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Message-ID: <ZOJuzakni1youMtX@shell.armlinux.org.uk>
Date:   Sun, 20 Aug 2023 20:51:41 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Jisheng Zhang <jszhang@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v4 2/9] net: stmmac: xgmac: add more feature
 parsing from hw cap

On Sun, Aug 20, 2023 at 09:15:06PM +0200, Andrew Lunn wrote:
> On Wed, Aug 16, 2023 at 11:29:19PM +0800, Jisheng Zhang wrote:
> > The XGMAC_HWFEAT_GMIISEL bit also indicates whether support 10/100Mbps
> > or not.
> 
> The commit message fails to explain the 'Why?' question. GMII does
> normally imply 10/100/1000, so i would expect dma_cap->mbps_1000 also
> implies 10/100/1000? So why also set dma_cap->mbps_10_100?
> 
> Maybe a better change would be to modify:
> 
>         seq_printf(seq, "\t1000 Mbps: %s\n",
>                    (priv->dma_cap.mbps_1000) ? "Y" : "N");
> 
> to actually say 10/100/1000 Mbps? It does not appear this is used for
> anything other than debugfs?

Indeed, it also looks to me like mbps_1000 and mbps_10_100 are only
used to print things in the debugfs file, and do not have any effect
on the driver.

Moreover:

drivers/net/ethernet/stmicro/stmmac/dwmac4.h:#define GMAC_HW_FEAT_GMIISEL      BIT(1)
drivers/net/ethernet/stmicro/stmmac/common.h:#define DMA_HW_FEAT_GMIISEL       0x00000002       /* 1000 Mbps Support */
drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h:#define XGMAC_HWFEAT_GMIISEL    BIT(1)

Seems to be all the same bit, and:

drivers/net/ethernet/stmicro/stmmac/dwmac4.h:#define GMAC_HW_FEAT_MIISEL       BIT(0)
drivers/net/ethernet/stmicro/stmmac/common.h:#define DMA_HW_FEAT_MIISEL 0x00000001      /* 10/100 Mbps Support */

So, if everyone defines the first few bits of the hw_cap identically,
is there any point to decoding this separately in each driver? Couldn't
the debugfs "show" function just parse the hw_cap directly? Wouldn't it
make more sense to print MII / GMII rather than 10/100 and 1000 ?

It does bring up one last question though: if the driver makes no use
of these hw_cap bits, then is there any point in printing them in the
debugfs file?

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