linux-kernel - Re: [[PATCH] 8/9] DMA-UART-Driver-for-AST2500

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Date:   Fri, 19 Oct 2018 12:41:56 +0530
From:   "sudheer.v" <open.sudheer@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Vinod <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Russell King <linux@...linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Jiri Slaby <jslaby@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Michael Moese <mmoese@...e.de>,
        Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        dmaengine@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org,
        Sudheer V <sudheer.veliseti@...eedtech.com>,
        ShivahShankar Shakarnarayan rao 
        <shivahshankar.shankarnarayanrao@...eedtech.com>
Subject: Re: [[PATCH] 8/9] DMA-UART-Driver-for-AST2500

On Fri, Oct 19, 2018 at 10:32:24AM +1100, Benjamin Herrenschmidt wrote:
> On Thu, 2018-10-18 at 15:25 +0530, Vinod wrote:
> > 
> > > It's not a dmaengine driver. It's a serial UART driver that happens to
> > > use a dedicated DMA engine.
> > 
> > Then I see no reason for it to use dmaengine APIs. The framework allows
> > people to share a controller for many clients, but if you have dedicated
> > one then you may use it directly
> 
> Well... the engine is shared by a few UARTs, they have dedicated rings
> but there's a common set of regs for interrupt handling etc.
> 
> That said, I still think it could be contained within a UART driver,
> there's little benefit in adding the framework overhead, esp since
> these are really weak cores, any overhead will be felt.
> 
> Ben.
> 
> > > It's unclear whether it should be split into two drivers, or just have
> > > the serial driver directly use the dma engine since that engine is
> > > dedicated in HW to only work on those UARTs and nothing else...
> > > 
> > > Cheers,
> > > Ben.

Initially we wanted to have  a single driver,
however we had an informal discussion with one of the maintainer 
and based on the feedback, followed the Linux DMA and UART architecture.

If this seperate DMA-engine driver adds more overhead than benifit,
we will merge them into a single UART driver and resubmitt the patches.
Vinod,
      can this dma-controller driver sit under dma subsystem?.
      or better to move it under UART framework.

Thank you.
-- Sudheer