RE: [EXT] Re: [PATCH v3 2/2] serial: 8250_dw: add Ambarella CV75 quirks

Hi Andy,

Originally we submitted a dedicated Ambarella UART driver. During review,
Linus pointed out that the register layout is basically 8250-compatible
and suggested reusing the existing 8250 code instead of maintaining a
separate driver. That is how we ended up with the current approach of
binding through 8250_dw plus a small set of Ambarella quirks.

[1] https://lore.kernel.org/r/CAD++jLm6HzEU1TM-c=Vtjv3FJEBecVP7TyxSgq_Q0zF-YwT6NA@mail.gmail.com/

Best regards!
Long Zhao

Ambarella Shanghai Co., Ltd
-----Original Message-----
From: Andy Shevchenko <[email protected]> 
Sent: Friday, August 14, 2026 19:53
To: Long Zhao <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>; Jiri Slaby <[email protected]>; Rob Herring <[email protected]>; Krzysztof Kozlowski <[email protected]>; Conor Dooley <[email protected]>; Linus Walleij <[email protected]>; Ilpo Järvinen <[email protected]>; [email protected]; [email protected]; [email protected]
Subject: Re: [EXT] Re: [PATCH v3 2/2] serial: 8250_dw: add Ambarella CV75 quirks

On Fri, Aug 14, 2026 at 09:33:45AM +0000, Long Zhao wrote:
> From: Andy Shevchenko <[email protected]>
> Sent: Friday, August 14, 2026 16:56
> On Thu, Aug 13, 2026 at 06:35:13PM +0800, Long Zhao via B4 Relay wrote:
> 
> > Add CV75 platform data with an ETOI IER mask and skip-set-rate so 
> > the DesignWare 8250 driver can drive the Ambarella UART for console use.

> Thanks for the review. Agreed the commit message was too brief.

...

> > +	/* Keep variant-specific bits in the shadow across subsequent 
> > +updates. */
> 
> This says what code is doing. Instead it must say "why?" the code is doing this.
> 
> > +	up->ier |= d->pdata->ier_mask;
> > +	serial_port_out(p, UART_IER, up->ier);

...

> In case there is an HW bug, where can we get the errata, please? (For 
> a bare minimum add a reference to the Errata document you give to the 
> customers.)

> This is not a silicon erratum. The Ambarella UART is not Synopsys 
> DesignWare IP; it is an Ambarella 8250-compatible UART. We still use 
> the snps-dw-apb-uart binding / 8250_dw because the programming model 
> matches closely enough, with two quirks (ETOI IER bit and 
> SKIP_SET_RATE). I will expand the commit message and rewrite the 
> comment to explain why in the next revision.

Okay, what does it make to think that 8250_dw is a suitable choice?
What are the features that are repetitive would be if you use a custom 8250-based driver (using the generic 8250 code)?

--
With Best Regards,
Andy Shevchenko


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