linux-kernel - Re: [PATCH v3 13/15] can: grcan: Add CANFD TX support alongside legacy CAN

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Message-ID: <20260127-fancy-fast-bird-e7c4c3-mkl@pengutronix.de>
Date: Tue, 27 Jan 2026 15:12:49 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Arun Muthusamy <arun.muthusamy@...sler.com>
Cc: robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, 
	mailhol@...nel.org, devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-can@...r.kernel.org
Subject: Re: [PATCH v3 13/15] can: grcan: Add CANFD TX support alongside
 legacy CAN

On 27.01.2026 15:06:55, Arun Muthusamy wrote:
>
> On 1/23/26 15:25, Marc Kleine-Budde wrote:
> > On 22.01.2026 13:10:36, Arun Muthusamy wrote:
> > > Include CANFD TX support with the legacy CAN support, enabling
> > > support for extended data payloads to provide higher bit rates.
> > >
> > > Signed-off-by: Arun Muthusamy <arun.muthusamy@...sler.com>
> > > ---
> > >   drivers/net/can/grcan.c | 102 +++++++++++++++++++++++++++++-----------
> > >   1 file changed, 74 insertions(+), 28 deletions(-)
> > >
> > > diff --git a/drivers/net/can/grcan.c b/drivers/net/can/grcan.c
> > > index 9fc18064fab1..508ad5320cff 100644
> > > --- a/drivers/net/can/grcan.c
> > > +++ b/drivers/net/can/grcan.c
> > > @@ -174,6 +174,7 @@ struct grcan_registers {
> > >   #define GRCAN_IRQ_DEFAULT (GRCAN_IRQ_RX | GRCAN_IRQ_TX | GRCAN_IRQ_ERRORS)
> > >
> > >   #define GRCAN_MSG_SIZE		16
> > > +#define GRCAN_CLASSIC_DATA_SIZE 8
> > >
> > >   #define GRCAN_MSG_IDE		0x80000000
> > >   #define GRCAN_MSG_RTR		0x40000000
> > > @@ -195,6 +196,10 @@ struct grcan_registers {
> > >   #define GRCAN_MSG_OFF		0x00000002
> > >   #define GRCAN_MSG_PASS		0x00000001
> > >
> > > +#define GRCAN_MSG_EID_MASK      GENMASK(28, 0)
> > > +#define GRCAN_MSG_BID_MASK      GENMASK(28, 18)
> > > +#define GRCAN_MSG_DLC_MASK      GENMASK(31, 28)
> > > +
> > >   #define GRCAN_BUFFER_ALIGNMENT		1024
> > >   #define GRCAN_DEFAULT_BUFFER_SIZE	1024
> > >   #define GRCAN_VALID_TR_SIZE_MASK	0x001fffc0
> > > @@ -227,6 +232,9 @@ struct grcan_registers {
> > >   #define GRCANFD_FDBTR_PS2_BIT 5
> > >   #define GRCANFD_FDBTR_SJW_BIT 0
> > >
> > > +#define GRCAN_TX_BRS  BIT(25)
> > > +#define GRCAN_TX_FDF  BIT(26)
> > > +
> > >   /* Hardware capabilities */
> > >   struct grcan_hwcap {
> > >   	/* CAN-FD capable, indicates GRCANFD IP.
> > > @@ -1218,6 +1226,13 @@ static void grcan_transmit_catch_up(struct net_device *dev)
> > >   	spin_unlock_irqrestore(&priv->lock, flags);
> > >   }
> > >
> > > +static int grcan_numbds(int len)
> > > +{
> > > +	if (len <= GRCAN_CLASSIC_DATA_SIZE)
> > > +		return 1;
> > > +	return 1 + ((len - GRCAN_CLASSIC_DATA_SIZE + GRCAN_MSG_SIZE) / GRCAN_MSG_SIZE);
> > This looks strange, what is calculated here? Why is "<=
> > GRCAN_CLASSIC_DATA_SIZE" a special case?
>
> grcan_numbds() calculates the number of buffer descriptors needed based on the data length.
> The condition "len <= GRCAN_CLASSIC_DATA_SIZE" addresses cases where the data length fits within a one buffer descriptor.
> For data lengths greater than "GRCAN_CLASSIC_DATA_SIZE", it computes additional descriptors needed.

What happens if "len = GRCAN_CLASSIC_DATA_SIZE + GRCAN_MSG_SIZE"?

| return 1 + ((len - GRCAN_CLASSIC_DATA_SIZE + GRCAN_MSG_SIZE) / GRCAN_MSG_SIZE);

= 1 + ((GRCAN_CLASSIC_DATA_SIZE + GRCAN_MSG_SIZE - GRCAN_CLASSIC_DATA_SIZE + GRCAN_MSG_SIZE) / GRCAN_MSG_SIZE);
= 1 + ((                          GRCAN_MSG_SIZE -                         + GRCAN_MSG_SIZE) / GRCAN_MSG_SIZE);
= 1 + 2;
= 3

Is this correct?

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
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