Date:   Fri, 25 Nov 2022 22:20:18 +0530
From:   "Sireesh Kodali" <sireeshkodali1@...il.com>
To:     "Vinod Koul" <vkoul@...nel.org>
Cc:     <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <dmaengine@...r.kernel.org>,
        <~postmarketos/upstreaming@...ts.sr.ht>
Subject: Re: [PATCH v2 0/3] dmaengine: Add support for immediate commands

On Mon Nov 14, 2022 at 3:53 AM IST, Vinod Koul wrote:
> On 11-11-22, 10:42, Sireesh Kodali wrote:
> > On Fri Nov 4, 2022 at 6:10 PM IST, Vinod Koul wrote:
> > > On 27-10-22, 10:44, Sireesh Kodali wrote:
> > > > The IPA v2.x block, found on some older Qualcomm SoCs, uses BAM DMA to
> > > > send and receive packets from the AP. It also uses BAM to receive
> > > > commands from the AP (and possibly the modem). These commands are
> > > > encoded as "Immediate Commands". They vary from regular BAM DMA
> > > > commands. Adding support for immediate commands is trivial, but requires
> > > > also adding Immediate Commands to the dmaengine API, which is what this
> > > > patch series does.
> > >
> > > Can you explain a bit more. I understand you need "Immediate Commands"
> > > but am really reluctant to add another interface to support a specific
> > > use case
> > >
> > 
> > Apologies for the delayed response
> > 
> > BAM supports both regular commands, and "immediate commands". Currently,
> > commands are used by the Qualcom NAND chip driver, while "immediate
> > commands" are intended to be used by the (yet to be mainlined) IPA
> > driver. From the BAM driver perspective, both immediate and regular
> > commands are simply a matter of setting the appropriate flag in the
> > descriptor. I don't have access to the documentation on BAM to know
> > exactly how these two modes differ, however I do know they are not
> > interchangable. If a different API is suggested, I can change the
> > implementation as needed.
>
> Ok, can you please explain what is meant by 'regular' cmd and
> 'immediate', lets see what is required here

Stephan pointed out the APQ8016E TRM has details on BAM. As I understand
it, 'regular' commands are queued register read/writes for the
peripheral. Immediate commands on the other hand seem to be interpreted
by the peripheral's firmware, and don't involve any register
writes/reads from BAM's perspective.

This is what the TRM has to say:

> Immediate (IMM) (only for BAM-NDP): Allows the software to create
> descriptors of type immediate, which does not generate any data
> transmissions or registers configuration, it is simply supplied to the
> peripheral, the peripheral then parses its fields (which are
> irrelevant to the BAM). Only the flags of this descriptor are relevant
> to the BAM, address and size are irrelevant, and BAM simply passes
> them as is to the peripheral. This can be used for the software to
> operate peripheral-specific operations within regular data operations.
> Immediate descriptors are published on the sidebands as 1 byte size
> descriptor, once BAM_NDP fetches an immediate descriptor, it publishes
> all recently fetched descriptors including the immediate descriptor
> with immediate indication, to inform the peripheral that the last
> published descriptor was immediate descriptor.

> Command (CMD) (only for BAM-lite and BAM-NDP): Allows the software to
> create descriptors of type command. Descriptors of type command do not
> generate any data transmissions but configure registers in the
> peripheral (write and read registers operations)

Regards,
Sireesh
>
> -- 
> ~Vinod

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