linux-kernel - RE: [PATCH v4 06/11] mtd: intel-dg: align 64bit read and write

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Message-ID: <CY5PR11MB636623DF234A109B17CD075AED142@CY5PR11MB6366.namprd11.prod.outlook.com>
Date: Thu, 2 Jan 2025 13:56:18 +0000
From: "Usyskin, Alexander" <alexander.usyskin@...el.com>
To: David Laight <david.laight.linux@...il.com>
CC: Miquel Raynal <miquel.raynal@...tlin.com>, Richard Weinberger
	<richard@....at>, Vignesh Raghavendra <vigneshr@...com>, "De Marchi, Lucas"
	<lucas.demarchi@...el.com>, Thomas Hellström
	<thomas.hellstrom@...ux.intel.com>, "Vivi, Rodrigo" <rodrigo.vivi@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
	<mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
	<airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Jani Nikula
	<jani.nikula@...ux.intel.com>, Joonas Lahtinen
	<joonas.lahtinen@...ux.intel.com>, Tvrtko Ursulin <tursulin@...ulin.net>,
	"Poosa, Karthik" <karthik.poosa@...el.com>, "Abliyev, Reuven"
	<reuven.abliyev@...el.com>, "Weil, Oren jer" <oren.jer.weil@...el.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 06/11] mtd: intel-dg: align 64bit read and write

> > GSC NVM controller HW errors on quad access overlapping 1K border.
> > Align 64bit read and write to avoid readq/writeq over 1K border.
> >
> > Acked-by: Miquel Raynal <miquel.raynal@...tlin.com>
> > Signed-off-by: Alexander Usyskin <alexander.usyskin@...el.com>
> > ---
> >  drivers/mtd/devices/mtd-intel-dg.c | 35
> ++++++++++++++++++++++++++++++
> >  1 file changed, 35 insertions(+)
> >
> > diff --git a/drivers/mtd/devices/mtd-intel-dg.c b/drivers/mtd/devices/mtd-
> intel-dg.c
> > index 76ef7198fff8..230bf444b7fe 100644
> > --- a/drivers/mtd/devices/mtd-intel-dg.c
> > +++ b/drivers/mtd/devices/mtd-intel-dg.c
> > @@ -238,6 +238,24 @@ static ssize_t idg_write(struct intel_dg_nvm *nvm,
> u8 region,
> >  		len_s -= to_shift;
> >  	}
> >
> > +	if (!IS_ALIGNED(to, sizeof(u64)) &&
> > +	    ((to ^ (to + len_s)) & GENMASK(31, 10))) {
> 
> I'm sure that should be (to + len_s - 1).
> Using GENMASK(31, 10) completely fails to indicate what is being tested.

Will look at it, but this form works fine in practice.
> 
> > +		/*
> > +		 * Workaround reads/writes across 1k-aligned addresses
> > +		 * (start u32 before 1k, end u32 after)
> > +		 * as this fails on hardware.
> > +		 */
> > +		u32 data;
> > +
> > +		memcpy(&data, &buf[0], sizeof(u32));
> 
> 	get_unaligned_u32()
> 

Can't find such function in kernel at all

> > +		idg_nvm_write32(nvm, to, data);
> > +		if (idg_nvm_error(nvm))
> > +			return -EIO;
> > +		buf += sizeof(u32);
> > +		to += sizeof(u32);
> > +		len_s -= sizeof(u32);
> > +	}
> 
> It isn't at all obvious why copying 4 bytes helps.
> Indeed, if 'to' is 1023 and 'len_s' is 2 it goes terribly wrong.

Data is aligned to sizeof(u32) before this check, so 1023 and 2 can't be here.
We can come here only if we 4bytes before 1k boundary and
we are reading all that left before boundary.
After that the data will be sizeof(u64) aligned.

> 
> 	David
> 
> > +
> >  	len8 = ALIGN_DOWN(len_s, sizeof(u64));
> >  	for (i = 0; i < len8; i += sizeof(u64)) {
> >  		u64 data;

- - 
Thanks,
Sasha