linux-kernel - Re: [PATCH v2 2/2] drivers: remove force dma flag from buses

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Date:   Wed, 21 Mar 2018 18:49:24 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Nipun Gupta <nipun.gupta@....com>
Cc:     "robin.murphy@....com" <robin.murphy@....com>,
        "hch@....de" <hch@....de>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "zajec5@...il.com" <zajec5@...il.com>,
        "andy.gross@...aro.org" <andy.gross@...aro.org>,
        "david.brown@...aro.org" <david.brown@...aro.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vinod.koul@...el.com" <vinod.koul@...el.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "frowand.list@...il.com" <frowand.list@...il.com>,
        "jarkko.sakkinen@...ux.intel.com" <jarkko.sakkinen@...ux.intel.com>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
        "dmitry.torokhov@...il.com" <dmitry.torokhov@...il.com>,
        "johan@...nel.org" <johan@...nel.org>,
        "msuchanek@...e.de" <msuchanek@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-soc@...r.kernel.org" <linux-soc@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bharat Bhushan <bharat.bhushan@....com>,
        Leo Li <leoyang.li@....com>
Subject: Re: [PATCH v2 2/2] drivers: remove force dma flag from buses

On Wed, Mar 21, 2018 at 04:28:46PM +0000, Nipun Gupta wrote:
> 
> 
> > -----Original Message-----
> > From: Greg KH [mailto:gregkh@...uxfoundation.org]
> > Sent: Wednesday, March 21, 2018 15:05
> > To: Nipun Gupta <nipun.gupta@....com>
> > Cc: robin.murphy@....com; hch@....de; linux@...linux.org.uk;
> > m.szyprowski@...sung.com; bhelgaas@...gle.com; zajec5@...il.com;
> > andy.gross@...aro.org; david.brown@...aro.org; dan.j.williams@...el.com;
> > vinod.koul@...el.com; thierry.reding@...il.com; robh+dt@...nel.org;
> > frowand.list@...il.com; jarkko.sakkinen@...ux.intel.com;
> > rafael.j.wysocki@...el.com; dmitry.torokhov@...il.com; johan@...nel.org;
> > msuchanek@...e.de; linux-kernel@...r.kernel.org; iommu@...ts.linux-
> > foundation.org; linux-wireless@...r.kernel.org; linux-arm-
> > msm@...r.kernel.org; linux-soc@...r.kernel.org; dmaengine@...r.kernel.org;
> > dri-devel@...ts.freedesktop.org; linux-tegra@...r.kernel.org;
> > devicetree@...r.kernel.org; linux-pci@...r.kernel.org; Bharat Bhushan
> > <bharat.bhushan@....com>; Leo Li <leoyang.li@....com>
> > Subject: Re: [PATCH v2 2/2] drivers: remove force dma flag from buses
> > 
> > On Wed, Mar 21, 2018 at 12:25:23PM +0530, Nipun Gupta wrote:
> > > With each bus implementing its own DMA configuration callback,
> > > there is no need for bus to explicitly have force_dma in its
> > > global structure. This patch modifies of_dma_configure API to
> > > accept an input parameter which specifies if implicit DMA
> > > configuration is required even when it is not described by the
> > > firmware.
> > 
> > Having to "remember" what that bool variable means on the end of the
> > function call is a royal pain over time, right?
> > 
> > Why not just create a new function:
> > 	dma_common_configure_force(dma)
> > that always does this?  Leave "dma_common_configure()" alone, and then
> > wrap the old code with these two helper functions that call the 'core'
> > code with the bool set properly?
> > 
> > That way you do not have to "know" what that parameter is, the function
> > name just documents it automatically, so when you see it in the
> > bus-specific code, no need to go and have to hunt for anything.  And if
> > you are reading the dma core code, it's obvious what is happening as the
> > functions are all right there.
> 
> How about we do not pass any flag in 'dma_common_configure()', and inside this
> API we pass "true" to 'of_dma_configure()'? I am saying this because currently
> both the busses (platform and AMBA) which uses 'dma_common_configure()' passes
> "true" value. If we create additional 'dma_common_configure_force()', then
> 'dma_common_configure()' will not be used anytime and will become redundant.
> 
> If someday new busses come and they needs to use similar functionality which
> 'dma_common_configure()' provides, but with passing "false" to 'of_dma_configure()',
> then what you suggests of having two separate such API's will be more reasonable
> and can be implemented?

If that makes things "simple", yes, sounds good.

greg k-h