linux-kernel - Re: [PATCH] serial: 8250_platform: Enable generic 16550A platform devices

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Message-ID: <ZumLC9I1Jf82Y8T-@smile.fi.intel.com>
Date: Tue, 17 Sep 2024 16:58:35 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Palmer Dabbelt <palmer@...belt.com>
Cc: Sunil V L <sunilvl@...tanamicro.com>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, linux-riscv@...ts.infradead.org,
	Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
	andrei.warkentin@...el.com, apatel@...tanamicro.com,
	ajones@...tanamicro.com, dfustini@...storrent.com
Subject: Re: [PATCH] serial: 8250_platform: Enable generic 16550A platform
 devices

On Tue, Sep 17, 2024 at 06:10:25AM -0700, Palmer Dabbelt wrote:
> On Mon, 29 Jul 2024 22:12:18 PDT (-0700), Sunil V L wrote:
> > Currently, 8250_platform driver is used only for devices with fixed
> > serial ports (plat_serial8250_port). Extend this driver for any generic
> > 16550A platform devices which can be probed using standard hardware
> > discovery mechanisms like ACPI.
> >
> > This is required in particular for RISC-V which has non-PNP generic
> > 16550A compatible UART that needs to be enumerated as ACPI platform
> > device.

> It seems a bit awkward that we need something RISC-V specific here.  I 
> guess the idea is that we don't want to depend on PNP because we'd end 
> up with a bunch of PCI dependencies as well?  That seems like kind of an 
> odd way to do things, but I'm frequently suprised by ACPI stuff so

We have the same for HSUART on Intel (see 8250_dw.c). The ACPI devices use
different approach than PNP. PNP has the exact requirements for compatibility
with the legacy UARTs (to some extend). It puts some limitations. I believe
having specifics for RISC-V is fine. But I haven't checked the real IP used
in the SoC. If it's one that we have the existing driver for (like again
mentioned 8250_dw.c), then it should be added there.

TL;DR: so, what is the IP is in hardware for UART? Custom?

-- 
With Best Regards,
Andy Shevchenko