netdev - Re: [PATCH v2 1/1] net: macb: Shorten max_tx_len to 4KiB

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Date:   Fri, 21 Apr 2023 17:39:52 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Simon Horman <simon.horman@...igine.com>
Cc:     Conor Dooley <conor.dooley@...rochip.com>,
        Jakub Kicinski <kuba@...nel.org>, daire.mcnamara@...rochip.com,
        nicolas.ferre@...rochip.com, claudiu.beznea@...rochip.com,
        davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 1/1] net: macb: Shorten max_tx_len to 4KiB - 56 on mpfs

Hey Simon,

On Fri, Apr 21, 2023 at 04:05:19PM +0200, Simon Horman wrote:
> On Thu, Apr 20, 2023 at 08:18:35AM +0100, Conor Dooley wrote:
> > Jaukb, Simon,
> > 
> > On Wed, Apr 19, 2023 at 06:02:22PM -0700, Jakub Kicinski wrote:
> > > On Tue, 18 Apr 2023 16:28:25 +0200 Simon Horman wrote:
> > 
> > [readding the context]
> > 
> > > > > static const struct macb_config sama7g5_gem_config = {
> > > > > @@ -4986,8 +4985,17 @@ static int macb_probe(struct platform_device *pdev)
> > > > >       bp->tx_clk = tx_clk;
> > > > >       bp->rx_clk = rx_clk;
> > > > >       bp->tsu_clk = tsu_clk;
> > > > > -     if (macb_config)
> > > > > +     if (macb_config) {
> > > > > +             if (hw_is_gem(bp->regs, bp->native_io)) {
> > > > > +                     if (macb_config->max_tx_length)
> > > > > +                             bp->max_tx_length = macb_config->max_tx_length;
> > > > > +                     else
> > > > > +                             bp->max_tx_length = GEM_MAX_TX_LEN;
> > > > > +             } else {
> > > > > +                     bp->max_tx_length = MACB_MAX_TX_LEN;
> > > > > +             }
> > 
> > > > no need to refresh the patch on my account.
> > > > But can the above be simplified as:
> > > > 
> > > >                if (macb_is_gem(bp) && hw_is_gem(bp->regs, bp->native_io))
> > > >                        bp->max_tx_length = macb_config->max_tx_length;
> > > >                else
> > > >                        bp->max_tx_length = MACB_MAX_TX_LEN;
> > > 
> > > I suspect that DaveM agreed, because patch is set to Changes Requested
> > > in patchwork :) 
> > > 
> > > Daire, please respin with Simon's suggestion.
> > 
> > I'm feeling a bit stupid reading this suggestion as I am not sure how it
> > is supposed to work :(

> just to clarify, my suggestion was at a slightly higher level regarding
> the arrangement of logic statements:
> 
> 	if (a)
> 		if (b)
> 
> 	vs
> 
> 	if (a && b)

Ah, I do at least feel less stupid now!
There are 3 possible conditions though, you'd be left with something
like:
	if !hw_is_gem()
	else if macb_config->max_tx_length
	else
> 
> I think your concerns are deeper and, in my reading of them, ought
> to be addressed.
> 
> > Firstly, why macb_is_gem() and hw_is_gem()? They both do the same thing,
> > except last time around we established that macb_is_gem() cannot return
> > anything other than false at this point.
> > What have I missed here?
> > 
> > Secondly, is it guaranteed that macb_config::max_tx_length is even
> > set?

These two were concerns about your suggestion, so they can now be
disregarded as you'd not been seriously suggesting that particular
if (false && hw_is_gem()) test ;)

> > Also, another question...
> > Is it even possible for `if (macb_config)` to be false?
> > Isn't it either going to be set to &default_gem_config or to
> > match->data, no? The driver is pretty inconsistent about if it checks
> > whether macb_config is non-NULL before accessing it, but from reading
> > .probe, it seems to be like it is always set to something valid at this
> > point.

This one though is more of a question for the drivers's maintainers -
Daire's only gone and copied what's done about 4 lines above the top of
the diff. Removing useless NULL checks, assuming they are useless, is
surely out of scope for sorting out this erratum though, no?

Cheers,
Conor.

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