linux-kernel - RE: [PATCH 3/4] serial: 8250: Add Aspeed UART driver

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Date:   Mon, 13 Feb 2023 04:18:16 +0000
From:   ChiaWei Wang <chiawei_wang@...eedtech.com>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "joel@....id.au" <joel@....id.au>,
        "andrew@...id.au" <andrew@...id.au>,
        "jirislaby@...nel.org" <jirislaby@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>
Subject: RE: [PATCH 3/4] serial: 8250: Add Aspeed UART driver

> From: Paul Menzel <pmenzel@...gen.mpg.de>
> Sent: Friday, February 10, 2023 6:35 PM
> 
> Dear Chia-Wei,
> 
> 
> Thank you for your patch.
> 
> Am 10.02.23 um 08:26 schrieb Chia-Wei Wang:
> > Add the driver for Aspeed UART/VUART devices, which are 16550A
> > compatible. It is an wrapper to cover the generic 16550A operation
> 
> a wrapper
> 
> > while exetending DMA feature for the devices.
> 
> extending
> 
> Is this for all ASPEED devices or only current ones?

There is major design change of UDMA.
Therefore, this patch is for AST2600 only.

> 
> How did you test this? What is the maximum transfer speed?

Yes.
I can't remember the precise data. The experiment was done like 2 years ago.
But it should be above 2MB/s

> 
> There are other serial drivers also supporting DMA, and those seem to use
> DMAengine?
> 
>      $ git grep dmaengine drivers/tty/serial/8250/
> 

Yes.
The UDMA design is dedicated to, and tightly coupled with each UART/VUART devices.
And UDMA does not support common DMA control such as suspend/resume.

We found certain discussions in the old mailing list saying that if a DMA controller is not fully fit into the DMAEngine subsystem,
placing the implementation in the soc folder might be an option.

Regards,
Chiawei