Re: [RFC v1 3/4] swiotlb: Allow dynamic allocation of bounce buffers

[Robin Murphy <robin.murphy@arm.com>]

On 2023-04-21 14:03, Petr Tesařík wrote:
> Hi Christoph!
> 
> I'd like to follow up on this sub-thread:
> 
> On Fri, 7 Apr 2023 12:15:55 +0200
> Petr Tesařík <petr@tesarici.cz> wroe:
> 
>> On Fri, 7 Apr 2023 07:57:04 +0200
>> Christoph Hellwig <hch@lst.de> wrote:
>> [...]
>>> (Btw, in case anyone is interested, we really need to get started
>>> on moving the dma fields out of struct device into a sub-struct
>>> only allocated for DMA capable busses)
>>
>> I like this idea. In fact, my WIP topic branch now moves the swiotlb
>> fields into a separate struct,
> 
> As you have noticed, I have removed that commit again in v2.
> 
> The reason is that I'm not sure about the intended goal. I have looked
> around for examples of moving fields out of struct device and found
> different approaches:
> 
> A. struct dev_msi_info
>     The MSI fields are merely grouped in a separate struct, which is
>     defined in device.h and embedded in struct device. I don't see much
>     benefit.
> 
> B. struct dev_pm_info
>     This struct is also embedded in struct device, but it is defined in
>     <linux/pm.h>, which is mentioned in MAINTAINERS. The benefit is that
>     further changes are reviewed by this maintainer. The downside is
>     that device.h includes pm.h.
> 
> C. struct dev_pin_info
>     This struct is merely declared in device.h and defined
>     pinctrl/devinfo.h (which is not included). Only a pointer to this
>     struct is stored in struct device. Of course, the pointer must be
>     initialized (and released) somehow.
> 
> Here my question: What did you want for DMA fields?
> 
> A. Only grouping those fields in their own struct?
> B. Or move the definition to another include file (cf. MAINTAINERS)?
> C. Or store a pointer in struct device?

dev->dma_parms is already this, and IIRC still has some very old 
comments somewhere about consolidating the other DMA-related fields in 
there.

> Since you mentioned "allocated", it sounds like you want to achieve C,
> but:
> 
> 1. Is it worth the extra dereference for every use?
> 2. How should the struct be allocated? Presumably not with kmalloc() in
>     device_initialize(), because I don't know how to determine if a
>     device is DMA capable this low in the call stack. So, should it be
>     allocated together with the containing structure? AFAICS this would
>     mean changing nearly all device drivers...

The bus code knows whether it's a DMA-capable bus or not, and as such 
should already be providing a .dma_configure method and/or performing 
some initialisation of DMA fields. Many of the ones that would need to 
are already providing dma_parms, in fact.

Thanks,
Robin.

> 
> As you can see, I need some more guidance from you before I can start
> working on this. ;-)
> 
> Petr T