linux-kernel - Re: [PATCH v2 1/3] net: sfp: add workaround for Realtek RTL8672 and RTL9601C chips

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Message-ID: <20210107194549.GR1551@shell.armlinux.org.uk>
Date:   Thu, 7 Jan 2021 19:45:49 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Pali Rohár <pali@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Schreiber <tschreibe@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Marek Behún <kabel@...nel.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] net: sfp: add workaround for Realtek RTL8672 and
 RTL9601C chips

On Thu, Jan 07, 2021 at 06:19:23PM +0100, Andrew Lunn wrote:
> > -static int sfp_quirk_i2c_block_size(const struct sfp_eeprom_base *base)
> > +static bool sfp_id_needs_byte_io(struct sfp *sfp, void *buf, size_t len)
> >  {
> > -	if (!memcmp(base->vendor_name, "VSOL            ", 16))
> > -		return 1;
> > -	if (!memcmp(base->vendor_name, "OEM             ", 16) &&
> > -	    !memcmp(base->vendor_pn,   "V2801F          ", 16))
> > -		return 1;
> > +	size_t i, block_size = sfp->i2c_block_size;
> >  
> > -	/* Some modules can't cope with long reads */
> > -	return 16;
> > -}
> > +	/* Already using byte IO */
> > +	if (block_size == 1)
> > +		return false;
> 
> This seems counter intuitive. We don't need byte IO because we are
> doing btye IO? Can we return True here?

It is counter-intuitive, but as this is indicating whether we need to
switch to byte IO, if we're already doing byte IO, then we don't need
to switch.

> > -static void sfp_quirks_base(struct sfp *sfp, const struct sfp_eeprom_base *base)
> > -{
> > -	sfp->i2c_block_size = sfp_quirk_i2c_block_size(base);
> > +	for (i = 1; i < len; i += block_size) {
> > +		if (memchr_inv(buf + i, '\0', block_size - 1))
> > +			return false;
> > +	}
> 
> Is the loop needed?

I think you're not reading the code very well. It checks for bytes at
offset 1..blocksize-1, blocksize+1..2*blocksize-1, etc are zero. It
does _not_ check that byte 0 or the byte at N*blocksize is zero - these
bytes are skipped. In other words, the first byte of each transfer can
be any value. The other bytes of the _entire_ ID must be zero.

> I also wonder if on the last iteration of the loop you go passed the
> end of buf? Don't you need a min(block_size -1, len - i) or
> similar?

The ID is 64 bytes long, and is fixed. block_size could be a non-power
of two, but that is highly unlikely. block_size will never be larger
than 16 either.

-- 
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