Hi Christoph! I'd like to follow up on this sub-thread: On Fri, 7 Apr 2023 12:15:55 +0200 Petr Tesařík <petr@xxxxxxxxxxx> wroe: > On Fri, 7 Apr 2023 07:57:04 +0200 > Christoph Hellwig <hch@xxxxxx> 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? 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... As you can see, I need some more guidance from you before I can start working on this. ;-) Petr T