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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Coquelin , Alexandre Torgue , Shawn Lin , Lorenzo Pieralisi , Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= , Manivannan Sadhasivam , Rob Herring , Bjorn Helgaas , Chanwoo Choi , MyungJoo Ham , Kyungmin Park , Qin Jian , Michael Turquette , Stephen Boyd , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , Jani Nikula , kernel@collabora.com, linux-kernel@vger.kernel.org, linux-mmc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-phy@lists.infradead.org, linux-sound@vger.kernel.org, netdev@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com, linux-pci@vger.kernel.org, linux-pm@vger.kernel.org, linux-clk@vger.kernel.org, llvm@lists.linux.dev, Tvrtko Ursulin Subject: Re: [PATCH 01/20] bitfield: introduce HWORD_UPDATE bitfield macros Message-ID: References: <20250612-byeword-update-v1-0-f4afb8f6313f@collabora.com> <20250612-byeword-update-v1-1-f4afb8f6313f@collabora.com> <5493fd6017de3f393f632125fad95945d1c4294c@intel.com> <3683577.irdbgypaU6@workhorse> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3683577.irdbgypaU6@workhorse> On Fri, Jun 13, 2025 at 01:55:54PM +0200, Nicolas Frattaroli wrote: > Hello, > > On Friday, 13 June 2025 10:51:15 Central European Summer Time Jani Nikula wrote: > > On Thu, 12 Jun 2025, Nicolas Frattaroli wrote: > > > Hardware of various vendors, but very notably Rockchip, often uses > > > 32-bit registers where the upper 16-bit half of the register is a > > > write-enable mask for the lower half. > > > > > > This type of hardware setup allows for more granular concurrent register > > > write access. > > > > > > Over the years, many drivers have hand-rolled their own version of this > > > macro, usually without any checks, often called something like > > > HIWORD_UPDATE or FIELD_PREP_HIWORD, commonly with slightly different > > > semantics between them. > > > > > > Clearly there is a demand for such a macro, and thus the demand should > > > be satisfied in a common header file. > > > > > > Add two macros: HWORD_UPDATE, and HWORD_UPDATE_CONST. The latter is a > > > version that can be used in initializers, like FIELD_PREP_CONST. The > > > macro names are chosen to not clash with any potential other macros that > > > drivers may already have implemented themselves, while retaining a > > > familiar name. > > > > > > Signed-off-by: Nicolas Frattaroli > > > --- > > > include/linux/bitfield.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++ > > > 1 file changed, 47 insertions(+) > > > > > > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h > > > index 6d9a53db54b66c0833973c880444bd289d9667b1..b90d88db7405f95b78cdd6f3426263086bab5aa6 100644 > > > --- a/include/linux/bitfield.h > > > +++ b/include/linux/bitfield.h > > > @@ -8,6 +8,7 @@ > > > #define _LINUX_BITFIELD_H > > > > > > #include > > > +#include > > > #include > > > #include > > > > > > @@ -142,6 +143,52 @@ > > > (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) \ > > > ) > > > > > > +/** > > > + * HWORD_UPDATE() - prepare a bitfield element with a mask in the upper half > > > + * @_mask: shifted mask defining the field's length and position > > > + * @_val: value to put in the field > > > + * > > > + * HWORD_UPDATE() masks and shifts up the value, as well as bitwise ORs the > > > + * result with the mask shifted up by 16. > > > + * > > > + * This is useful for a common design of hardware registers where the upper > > > + * 16-bit half of a 32-bit register is used as a write-enable mask. In such a > > > + * register, a bit in the lower half is only updated if the corresponding bit > > > + * in the upper half is high. > > > + */ > > > +#define HWORD_UPDATE(_mask, _val) \ > > > + ({ \ > > > + __BF_FIELD_CHECK(_mask, ((u16) 0U), _val, \ > > > + "HWORD_UPDATE: "); \ > > > + (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask)) | \ > > > + ((_mask) << 16); \ > > > + }) > > > > i915 uses something like this for a few registers too, with the name > > _MASKED_FIELD(). I think we could use it. > > > > I do think this is clearly an extension of FIELD_PREP(), though, and > > should be be named similarly, instead of the completely deviating > > HWORD_UPDATE(). > > > > Also, we recently got GENMASK() versions with sizes, GENMASK_U16() > > etc. so I find it inconsistent to denote size here with HWORD. > > > > FIELD_PREP_MASKED_U16? MASKED_FIELD_PREP_U16? Something along those > > lines? > > Yeah, I agree the name could be better. I used HWORD_UPDATE as Yury and > I couldn't come up with a name we liked either, and Yury suggested not > breaking from what's already there too much. I do think making the name > more field-adjacent would be good though, as well as somehow indicating > that it is 16 bits of data. I suggested a wonderful name that explains everything. Didn't I? It has the only problem - it's 25 chars long. :) So yeah, let's think once more about a better _short_ name, or just stick to the existing naming scheme. > > And perhaps that (and more potential users) could persuade Jakub that > > this is not that weird after all? > > I will operate under the assumption that Jakub's opinion will not change > as he ignored the commit message that talks about multiple vendors, > ignored the cover letter that talks about multiple vendors, and ignored > my e-mail where I once again made it clear to him that it's multiple > vendors, and still claims it's a Rockchip specific convention. As far as I understood, he concerns not about number of drivers that opencode HIWORD_UPDATE(), but that this macro is not generic enough to live in bitfield.h. And it's a valid concern - I doubt it will be helpful somewhere in core and arch files. I think that creating a separate header like hw_bitfield.h, or hw_bits.h aimed to absorb common helpers of that sort, would help to reach the strategic goal - decreasing the level of code duplication in the driver swamp. Thanks, Yury