linux-kernel - Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro

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Message-ID: <2025092924-cork-mobilize-cfbf@gregkh>
Date: Mon, 29 Sep 2025 21:37:44 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	dakr@...nel.org, acourbot@...dia.com,
	Alistair Popple <apopple@...dia.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	bjorn3_gh@...tonmail.com, Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	John Hubbard <jhubbard@...dia.com>, Timur Tabi <ttabi@...dia.com>,
	joel@...lfernandes.org, Elle Rhumsaa <elle@...thered-steel.dev>,
	Yury Norov <yury.norov@...il.com>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code
 from register! into new macro

On Mon, Sep 29, 2025 at 03:26:57PM -0400, Joel Fernandes wrote:
> On 9/24/2025 12:40 PM, Greg KH wrote:
> > On Tue, Sep 23, 2025 at 06:24:34PM -0400, Joel Fernandes wrote:
> [..]
> > 
> >> For the separate issue of enforcing endianness with respect to (across)
> >> multiple fields, I agree with you that if the user's backend (the consumer of
> >> the data) is not doing such conversion, say via regmap, then that becomes a
> >> problem. But that problem is orthogonal/different and cannot be solved here.  
> > 
> > But that is exactly what these macros are being defined here for, so to
> > ignore that is going to cause problems :)
> > 
> 
> If needed, happy to add endianness support as needed by providing additional
> options to the macro. Based on this thread, it sounds like we want see if that
> is really needed here or can be solved elsewhere (?). The mental model I kind of
> have is this macro should only be dealing with CPU native endianness, much like
> bitfields in C deal with CPU endianness. Hmm.

Just don't go down the old path like drivers/net/fddi/skfp/h/supern_2.h
does with it's definition of:

	union tx_descr {
		struct {
	#ifdef  LITTLE_ENDIAN
		unsigned int    tx_length:16 ;  /* frame length lower/upper byte */
		unsigned int    tx_res   :8 ;   /* reserved      (bit 16..23) */
		unsigned int    tx_xmtabt:1 ;   /* transmit abort */
		unsigned int    tx_nfcs  :1 ;   /* no frame check sequence */
		unsigned int    tx_xdone :1 ;   /* give up token */
		unsigned int    tx_rpxm  :2 ;   /* byte offset */
		unsigned int    tx_pat1  :2 ;   /* must be TXP1 */
		unsigned int    tx_more  :1 ;   /* more frame in chain */
	#else
		unsigned int    tx_more  :1 ;   /* more frame in chain */
		unsigned int    tx_pat1  :2 ;   /* must be TXP1 */
		unsigned int    tx_rpxm  :2 ;   /* byte offset */
		unsigned int    tx_xdone :1 ;   /* give up token */
		unsigned int    tx_nfcs  :1 ;   /* no frame check sequence */
		unsigned int    tx_xmtabt:1 ;   /* transmit abort */
		unsigned int    tx_res   :8 ;   /* reserved      (bit 16..23) */
		unsigned int    tx_length:16 ;  /* frame length lower/upper byte */
	#endif
		} t ;
		long    i ;
	} ;

CPU endinness, hah, as if...