linux-kernel - Re: [PATCH V18 4/6] i2c: tegra: Add DMA support

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Date:   Mon, 11 Feb 2019 14:28:51 +0100
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com,
        mkarthik@...dia.com, smohammed@...dia.com, talho@...dia.com,
        peda@...ntia.se, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH V18 4/6] i2c: tegra: Add DMA support

Hi Dmitry,

> > I am not sure if you are aware of this document, so I mention it:
> > 
> > Documentation/i2c/DMA-considerations
> > 
> > I am not pushing you to use the i2c_get_dma_safe_msg_buf() helpers,
> > just wanted to make sure you know about them. I am also fine with an
> > incremental patch on top of this if you want to add usage of those
> > helpers somewhen later.
> 
> At least I was not aware of those helpers. It looks to me that the
> approach of having a statically allocated buffer is more optimal than
> having to allocate and map the buffer on each transfer. 

Can be argued, yes.

> >>  /* Match table for of_platform binding */
> >>  static const struct of_device_id tegra_i2c_of_match[] = {
> >>  	{ .compatible = "nvidia,tegra194-i2c", .data = &tegra194_i2c_hw, },
> >> +	{ .compatible = "nvidia,tegra186-i2c", .data = &tegra186_i2c_hw, },
> >>  	{ .compatible = "nvidia,tegra210-i2c", .data = &tegra210_i2c_hw, },
> >>  	{ .compatible = "nvidia,tegra124-i2c", .data = &tegra124_i2c_hw, },
> >>  	{ .compatible = "nvidia,tegra114-i2c", .data = &tegra114_i2c_hw, },
> > 
> > Shouldn't this be a seperate patch?
> 
> I asked for this change and no, it shouldn't be a separate patch as it
> adds "has_apb_dma = false" property for T186 that older Tegra's have
> as "true". Without this change T186 uses tegra_i2c_hw_feature of T210,
> see [0]. Hence this change is absolutely correct and appropriate for
> this patch.

Then please update the patch description to contain this information.

> > This should definately be a seperate patch. While I am all for taking
> > it, are you sure it does not regress on older Tegra platforms?
> > 
> 
> All the patches are tested on older Tegra's (T20/T30 specifically by
> me) and we fixed several bugs that were regressing them over the few
> versions of the patchset. I specifically asked for this change for
> older Tegra's because the APBDMA driver (T20-T210) is getting
> registered from the module-init level and hence I2C driver probe is
> always getting deferred. I'm not sure that it's worth to factor out
> this hunk into a separate patch as it's directly related to the DMA
> support addition, maybe worth to mention why this is needed in the
> commit message.

At least, this, yes.

Thanks,

   Wolfram


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