linux-kernel - Re: [PATCH 06/10] PCI/DPC: Use FIELD

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Message-ID: <f6c43aea-8e30-c3df-8318-3e9f79aaf6c@linux.intel.com>
Date:   Wed, 11 Oct 2023 14:13:47 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        Krzysztof Wilczyński <kw@...ux.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 06/10] PCI/DPC: Use FIELD_GET()

On Wed, 11 Oct 2023, Jonathan Cameron wrote:

> On Tue, 10 Oct 2023 15:44:32 -0500
> Bjorn Helgaas <helgaas@...nel.org> wrote:
> 
> > From: Bjorn Helgaas <bhelgaas@...gle.com>
> > 
> > Use FIELD_GET() to remove dependences on the field position, i.e., the
> > shift value.  No functional change intended.
> > 
> > Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
> A question about what 'rules' you are applying for using these macros
> vs choosing not not do so. Personally I prefer using them even for
> flag fields mostly because it makes the code more consistent and
> the compiler should remove any unnecessary shifts that result.
> 
> > ---
> 
> > diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> > index eeec1d6f9023..a9fdc2e3f110 100644
> > --- a/drivers/pci/quirks.c
> > +++ b/drivers/pci/quirks.c
> > @@ -6154,7 +6154,7 @@ static void dpc_log_size(struct pci_dev *dev)
> >  	if (!(val & PCI_EXP_DPC_CAP_RP_EXT))
> 
> This is what I'm commenting on below.
> 
> >  		return;
> >  
> > -	if (!((val & PCI_EXP_DPC_RP_PIO_LOG_SIZE) >> 8)) {
> > +	if (FIELD_GET(PCI_EXP_DPC_RP_PIO_LOG_SIZE, val) == 0) {
> 
> Why do this one and not the one just above?
> In both cases extracting a field then comparing it to 0, I'm not sure
> it makes sense to care if that field is 1 bit or multiple bit.

I cannot speak for Bjorn but at least I've left flag checks untouched
(but when pulling the flag into a variable, I've made it with FIELD_GET()).

In anycase, that seems minor issue though compared with defined values of 
the field being incompatible with the FIELD_GET()ed value (when the shift 
is non-zero). I wish there would be good solution to that but so far I've 
not come up anything that would be short and simple enough.

-- 
 i.