netdev - Re: [net-next 15/19] can: tcan4x5x: rework SPI access

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Message-ID: <20210111100321.3f9a0b08@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 11 Jan 2021 10:03:21 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, netdev@...r.kernel.org,
        linux-can@...r.kernel.org, Dan Murphy <dmurphy@...com>,
        kernel@...gutronix.de, Sean Nyekjaer <sean@...nix.com>,
        davem@...emloft.net
Subject: Re: [net-next 15/19] can: tcan4x5x: rework SPI access

On Mon, 11 Jan 2021 16:35:30 +0100 Ahmad Fatoum wrote:
> Hello Jakub,
> 
> On 08.01.21 17:32, Jakub Kicinski wrote:
> > On Fri, 8 Jan 2021 11:07:26 +0100 Ahmad Fatoum wrote:  
> >>>>> +struct __packed tcan4x5x_map_buf { 
> >>>>> +	struct tcan4x5x_buf_cmd cmd; 
> >>>>> +	u8 data[256 * sizeof(u32)]; 
> >>>>> +} ____cacheline_aligned;       
> >>>>
> >>>> Due to the packing of the struct tcan4x5x_buf_cmd it should have a length of 4
> >>>> bytes. Without __packed, will the "u8 data" come directly after the cmd?    
> >>>
> >>> Yup, u8 with no alignment attribute will follow the previous
> >>> field with no holes.    
> >>
> >> __packed has a documentation benefit though. It documents that the author
> >> considers the current layout to be the only correct one. (and thus extra
> >> care should be taken when modifying it).  
> > 
> > ____cacheline_aligned adds a big architecture dependent padding at the
> > end of this struct, so the size of this structure is architecture
> > dependent. Besides using packed forced the compiler to use byte by byte
> > loads on architectures without unaligned access, so __packed is not
> > free.  
> 
> https://godbolt.org/z/j68x8n
> 
> seems to indicate that explicit alignment "overrules" packed's implicit
> alignment of 1 as
>  there isn't any byte-by-byte access generated for a struct
> that is both packed and cacheline aligned. packed only structs are accessed
> byte-by-byte however.
> 
> Did I get something wrong in my testcase?
> 
> I compiled with ARM gcc 8.2  -mno-unaligned-access -fno-strict-aliasing -O2

I see, that's why I said combining ____cacheline_aligned with __packed
looks very confusing. Good to know which one takes precedence.
That doesn't change my recommendation to remove __packed, though, let's
not leave readers of this code scratching their heads.