linux-kernel - Re: [PATCH 02/13] spi: spi-fsl-lpspi: Set correct chip-select polarity bit

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Message-ID: <aKSF/mDIV2mKgArP@lizhi-Precision-Tower-5810>
Date: Tue, 19 Aug 2025 10:11:10 -0400
From: Frank Li <Frank.li@....com>
To: James Clark <james.clark@...aro.org>
Cc: Mark Brown <broonie@...nel.org>, Clark Wang <xiaoning.wang@....com>,
	Fugang Duan <B38611@...escale.com>, Gao Pan <pandy.gao@....com>,
	Fugang Duan <fugang.duan@....com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	Larisa Grigore <larisa.grigore@....nxp.com>,
	Larisa Grigore <larisa.grigore@....com>,
	Ghennadi Procopciuc <ghennadi.procopciuc@....com>,
	Ciprianmarian Costea <ciprianmarian.costea@....com>, s32@....com,
	linux-spi@...r.kernel.org, imx@...ts.linux.dev,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 02/13] spi: spi-fsl-lpspi: Set correct chip-select
 polarity bit

On Tue, Aug 19, 2025 at 09:21:08AM +0100, James Clark wrote:
>
>
> On 18/08/2025 4:19 pm, Frank Li wrote:
> > On Mon, Aug 18, 2025 at 02:05:16PM +0100, James Clark wrote:
> > >
> > >
> > > On 14/08/2025 5:49 pm, Frank Li wrote:
> > > > On Thu, Aug 14, 2025 at 05:06:42PM +0100, James Clark wrote:
> > > > > From: Larisa Grigore <larisa.grigore@....com>
> > > > >
> > > > > The driver currently supports multiple chip-selects, but only sets the
> > > > > polarity for the first one (CS 0). Fix it by setting the PCSPOL bit for
> > > > > the desired chip-select.
> > > > >
> > > > > Fixes: 5314987de5e5 ("spi: imx: add lpspi bus driver")
> > > > > Signed-off-by: Larisa Grigore <larisa.grigore@....com>
> > > > > Signed-off-by: James Clark <james.clark@...aro.org>
> > > > > ---
> > > > >    drivers/spi/spi-fsl-lpspi.c | 6 ++++--
> > > > >    1 file changed, 4 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/spi/spi-fsl-lpspi.c b/drivers/spi/spi-fsl-lpspi.c
> > > > > index d44a23f7d6c1..c65eb6d31ee7 100644
> > > > > --- a/drivers/spi/spi-fsl-lpspi.c
> > > > > +++ b/drivers/spi/spi-fsl-lpspi.c
> > > > > @@ -70,7 +70,7 @@
> > > > >    #define DER_TDDE	BIT(0)
> > > > >    #define CFGR1_PCSCFG	BIT(27)
> > > > >    #define CFGR1_PINCFG	(BIT(24)|BIT(25))
> > > > > -#define CFGR1_PCSPOL	BIT(8)
> > > > > +#define CFGR1_PCSPOL_MASK	GENMASK(11, 8)
> > > > >    #define CFGR1_NOSTALL	BIT(3)
> > > > >    #define CFGR1_HOST	BIT(0)
> > > > >    #define FSR_TXCOUNT	(0xFF)
> > > > > @@ -425,7 +425,9 @@ static int fsl_lpspi_config(struct fsl_lpspi_data *fsl_lpspi)
> > > > >    	else
> > > > >    		temp = CFGR1_PINCFG;
> > > > >    	if (fsl_lpspi->config.mode & SPI_CS_HIGH)
> > > > > -		temp |= CFGR1_PCSPOL;
> > > > > +		temp |= FIELD_PREP(CFGR1_PCSPOL_MASK,
> > > > > +				   BIT(fsl_lpspi->config.chip_select));
> > > > > +
> > > >
> > > > Feel like FILED_PREP(..., BIT()) is stranged.
> > > >
> > > > I suggest #define CFGR1_PCSPOL(x) BIT((x) + 8)
> > > >
> > > > Frank
> > >
> > > It's using an existing macro that everyone knows though and I found 65
> > > instances of exactly this. It can be read as "set bit X and put it into the
> > > PCSPOL field without any further investigation.
> >
> > Where have such pattern in kernel?
> >
> > Frank
> >
>
> Grep "FIELD_PREP\(.*,\n?.*BIT\("
>
> 65 results, e.g:
>
>   return FIELD_PREP(RTL8366RB_LED_0_X_CTRL_MASK, BIT(port));

Thanks.

Frank
>
> James
>
> >
> > > If we make a new macro, first the reader will have to jump to it, then it
> > > still doesn't immediately explain what the "+ 8" part is. Using FIELD_PREP()
> > > also has the potential to use autogenerated field masks from a machine
> > > readable version of the reference manual. You can't statically check your
> > > macro to see if + 8 is correct or not, and it also doesn't catch overflow
> > > errors like FIELD_PREP() does.
> > >
> > > There might be an argument to add a new global macro like FIELD_BIT(mask,
> > > bit). But it's not very flexible (can't set multiple bits) and you can
> > > already accomplish the same thing by adding BIT() to the existing one.
> > >
> > > Thanks
> > > James
> > >
> > > >
> > > > >    	writel(temp, fsl_lpspi->base + IMX7ULP_CFGR1);
> > > > >
> > > > >    	temp = readl(fsl_lpspi->base + IMX7ULP_CR);
> > > > >
> > > > > --
> > > > > 2.34.1
> > > > >
> > >
>